Hey ebbandflow.. I work at a computer store and on the side build customer computers..
With 2k you can do a lot of stuff..
mainly I would suggest the following if you want to build a really good audio rig..
Chassis: Something with a good amount of space up front in the 3.5' drive bays if you ever need to put stuff there. Case isn't as important as other things, but its always nice to have some room to play around with.
CPU: Core 2 Duo 6600 @ 2.4ghz. I have this cpu overclocked to 3.2ghz without breaking a sweat. It runs Vista and anything else I throw at it in stride.. the prices for all the intel chips are starting to go down. I paid almost 400CDN for my chip and now I can get one for just over $240

Great chip though for this sort of work.
Memory: 2gbs Minimum.. if you're making this a straight up audio machine, I would reocmmend 4gbs.. specially if you have a lot of prodcustion programs up. Doesn't have to be the fastest ram ever.. but you wanna have more quantity.
mobo: Anything from Asus or EVGA should suffice. Mobo I find is more important for gamers.
Sound Card: I would grab a really good low latency high functionality PCI audio card, maybe from RME or M-Audio for I/O. I want to grab the rme multiface but I still need a new video card lol
The rest of the money left over, you may need to spend on sample libraries, or better yet, software like NI stuff, and for a host sequencer if you don't already have once. Can't do much without those obviously.
Also, invest in a decent midi controller.. splurge a bit on a nice keyboard, since you don't need to spend an arm and a leg to get a top of the line PC in order to get great sound..
as metioned before though, do NOT waste your time and money grabbing a full unit from best buy or circuit city or anything like that. It's far more rewarding to build your own machine, and you get a much higher value in amassing the parts and doing it yourself.. plus you may learna thing or two
Hope this helped.. if you have any questions just pm me
