My two cents..
Since I'm in the process of buying a new PC for music-purposes, I'll give an alternative to the system above. Please... no flaming about my choices, there's an untold number of possible permutations of system, some better, many worse - but I stand by my decisions.. and so should you if you know your stuff :)
First, a case to fill up! There's lots of choices, but don't get cheap on the case, buy a bigtower (lots of space for harddisks, a good powersupply -> 340W minimum). I like the looks of the Chieftec Dragon, which should be about €140, but there's loads of good alternatives. Also, it has a kit with sound-absorbing mats available for about €60, which should REALLY make a difference in the sound picked up by any mike placed (too) close to the PC, and decreases the need for all sorts of clever silent cooling (which can get expensive!). Still, a water-cooled big-tower IS rather sexy :), sad but true.
--> Case: Chieftec Dragon ServerTower with acoustic dampening mats
The choice between the AMD Athlon XP and Intel Pentium 4 processors is not an easy one - basically, AMD offers more "bang for the buck" (performance-wise), where Intel offers lower power consumption and thus less heat. Personally, I'm going for an Intel system. The important first issue then becomes that you get the "new" Pentium 4 'B' type, which runs on a 533MHz clock instead of 400Mhz.
--> CPU: Pentium 4 2.53GHz. Cooling is included with the Box-version, and sufficient. If you use the recommended sound-absoring mats for the case, you might not need a silent fan solution. But if you do, there's plenty to choose from - I like the Zalman P4 Quiet CPU Flower Cooler (it looks so cute :)
Next: the motherboard. The choice of motherboard is very much determined by the choice of the chipset. If you want to avoid expensive and near-market-obselete RAM (variously called 'Rambus', Rimm, PC800 or RD-RAM - all the same thing), don't go for a motherboard based on the Intel 850 chipset - the combination of the 850 with RD-RAM may be the fastest combi, even Intel itself offers chipsets intended for DDR-RAM (also called Dimm) - this is the kind of memory formerly used on Athlon-based motherboards, and is thus widely available, relatively inexpensive, and fast.
Basically, for Intel the choice is between the Intel 845E and 845G chipsets - officially, there's only one difference between the two: the 'G' has onboard graphics. Unofficially, but definately, the 'G' also supports DDR-333Mhz RAM, whereas the the 'E' only supports DDR-266Mhz. Since the onboard-VGA is (for once) of quite good quality, I'd go for the 'G' with DDR-333.
There are many good motherboards based on the Intel 845G, but my personal choice is the Asus P4B533-V. If you wanna overclock (which I wouldn't.. reliability is WAY more important in an audio-PC than a few percent extra speed), I'd go for the EPoX 4G4A. Oh, and I'd shy away from motherboards with RAID - RAID is often not for the faint of heart to install and maintain (or so I'm told), and I wouldn't use it - even considering I'm an IT-professional.
--> Motherboard: Asus P4B533-V
At the moment, DDR 333Mhz RAM (also known as PC2700) is the memory of choice, until 400Mhz becomes widely available - but that will require an (as yet) unreleased chipset/motherboard!
Please don't remind me that you're technically overclocking the motherboard by running DDR-333 memory, I'm aware of it, but many sources have convinced me the DDR-333 support is as it was intented; I understand you can even find this speed in the manual of certain 845G-mainboards..
--> RAM: 1x512MB DDR-333 (PC2700) of a name-brand (e.g. Kingston, Dane-Elec), 2x512MB couldn't hurt of course :) and really isn't that expensive - don't buy 256MB Dimm's, you usually only have three slots, so max RAM is determined by the individual size of the Dimm's
The choice of harddrive can be very simple: the Western Digital WD1200JB drive - it's 120GB, has 8MB of cache, and has been tested to be one of (if not THE) fastests E-IDE drives. It's very popular, so widely available. Do get the 120GB version; there's also an 80GB version, but it is slightly slower. According to some, it's no longer really necessary to use seperate drives as system- and audio-drive, as was previously recommended, but two of these drive WILL make a difference in the number of audio-tracks you can use simultaneously (although, since we're already well passed maybe 50 simultaneous audio-tracks, one should wonder if this is that big a deal, except if you're gonna record 24bit/96kHz audio - in which case you need all the help you can get! :)
--> Harddrive: Western Digital WD1200JB 120GB/8MB cache
A CD-burner should be standard. Me? I'm going for an external USB2.0 burner - they're no less fast than their internal counterparts (thanks to USB2.0, which is MUCH faster than the currently popular USB1.1 standard). Why? To avoid using a master/slave configuration. Agreed, it's not really a big deal, but every bit helps - IDE performance is crucial for multitracking. But an internal model will be fine, especially if you've opted for the one harddrive and don't need the second IDE for a DVD-drive (why bother? you can always fit one when software REALLY comes on DVD's). Brand? Plextor has been the number one choice for many people for many years, and since prices are so low for burners, why not go for the leader?
--> Burner: Plextor PX-W4012TU (external)
The soundcard is of course a key component, and also IMO the major flaw of the system described above - the Soundblaster Audigy is really not a good choice. For example, the Terratec DMX6Fire24/96 has REAL 24bit/96kHz audio, many more connections than even the most expensive Audigy - including a dedicated PHONO-connection with preamp for connecting a turntable, a high-quality MIC-connection, standard MIDI-connectors, and both optical (TOSLINK) and RCA-digital connectors. It is far superior to the Audigy, and even slightly less expensive than the Soundblaster Audigy Platinum.
Of course, this card is NOT meant for multitracking - which for me is unimportant. If you wanna multitrack, there's so many factors to consider that I can't go into them in detail. A few pointers: check with your software-manufacturer of choice if the card is SPECIFICALLY supported, check if there are Win2000 drivers (see below), and check some of the websites below. This really can get very complex, so use the forums for questions of Brand A vs. Brand B. I had an Aardvark Aark 20/20 for about four years, and its sound was AMAZING compared even to current cards. But you really are spoiled for choice today.
--> Soundcard: Terratec DMX6Fire24/96
Well, you can get by with the integrated VGA on the above-mentioned motherboard - which is a first! For years, build-in VGA has been synonymous with crappy-VGA, but the Intel 845G has changed that (although gamers will, rightly, disagree). The built-in VGA of the Intel 845G is easily on par with a GeForce 2MX.. so you could even play a game of Wolfenstein if you wanna risk screwing up your carefully tweaked PC :)
--> Graphics: built-in!!!
And the last (very) expensive bit: a monitor. TFT is nice :), buy a 17" if you can afford it, but a 15" TFT will do. For music, big advantage over conventional monitors (CRT's) is the much reduced radiation - an CRT tends to increase the noise-level significantly of anything nearby. On the other hand, the price of 1 17" TFT will buy you two high-quality 19" CRT's!! Which could be significant - if you ignored my build-in VGA recommendation and actually bought a video-card with two monitor-outputs (in which case, look in to the Matrox Parhelia series). Me? I'd go for an Iiyama MA901U 19" CRT, which has a Diamondtron tube and is VERY appealing at 1280x1024, but I've already bought an Iiyama LS902UT 19" about four months ago. This is not bad either. But, given card blanche, the Iiyama AS4315UT 17" TFT is the one I'd go for. Yes, I like Iiyama! :) but I'm sure other A-brands are as good..
--> Monitor: Iiyama AS4315UT 17" TFT
Finally: diskdrive (whichever matches the case color :), keyboard (buy one you like), mouse (optical, but not wireless: wireless mice are heavy and drain bateries, Logitech is good, Microsoft is good).
Just for the hell of it, here is the entire system again:
Chieftec Dragon ServerTower with acoustic dampening mats, Asus P4B533-V motherboard, Pentium 4 'B' 2.53GHz, 2x Dane-Elec 512MB DDR-333, Western Digital WD1200JB 120GB/8MB cache, Terratec DMX6Fire24/96 soundcard, Iiyama AS4315UT 17" TFT, Plextor PX-W4012TU external USB2.0 burner, a diskdrive, a keyboard, a mouse
For an AMD Athlon XP system, substitute the motherboard with the MSI KT3 Ultra2, but any one with the KT333-chipset is good, and buy the fastest Athlon XP you can afford.
Choices, choices. If you're a diehard Windozer, go for Cakewalk Sonar - Cakewalk is and always was PC-software. Even with many years of IT-experience, I never could get my head around either Cubase or Logic, whereas with Cakewalk and Cakewalk Sonar, EVERYTHING is where I suspect it to be. It has advantages and disadvantages over Cubase, but this is a complex subject. Suffice to say, you can use VST/VSTi plugins in Sonar by using one of the many DX/VST "wrappers".. so the choice really is one of personal preferrences over interface and more arcane functions. As for plugins, check the site below and enjoy :)
Sound on Sound The ULTIMATE resource for anything related to homerecording, synths, and audio-PC's. This UK magazine is an absolute must-read, and an amazing repository of knowledge on ANY subject related to the modern (home-) studio. Really..
K-v-R is the best VSTi/DXi-plugin site I'm aware of. Start here to choose your virtual weaponry :)