There are bound to be some small computer shops around you that will assemble a computer for you at very little cost above those for parts. I'd look into one of these, and give them the exact specs you want. You can take the $2,000 you'll save and buy a nice sound card, software, and mics.
I agree with J -- the motherboard is CRITICAL. A great MoBo for one sound card may be crap for another. I would start with picking your card based on your needs (number of inputs, phantom power, reviews etc.), then check out the web page and/or call the tech guys of the sound card and get their recommendations for a motherboard. Beyond that look for:
High quality power supply (at least 300 w, but you may as well get 400 w). Enermax makes good quiet ones.
A large, 7200 RPM hard drive. Better yet, get 2: one for just ausio data, one for everything else (the non-audio one can be 5400 RPM). There are lots of opinions on which are best but Maxtor Daimond Max is a good bet. Tip: Comp USA brand hard drives are just repackaged Maxtor Daimond Maxs, and usally cheaper.
Video card: lots of recommendations on this board for the MAtrox g450. Dual head, and great 2-D graphics. Not great for gaming, however.