Go to a local computer store and have them build it for you, you're just not getting a good enough deal with the manufacturers you'd be looking at because of the shit they bundle in. Go for a fast, dual-core processor, 2 gigs of ram, a hard drive no smaller than 100GB (200 if you want to be working on several projects), and make sure your audio interface will be covered somewhere - either have them put it in if it's a sound card or make sure you have a Firewire port if it's something external. Fuck manufacturers. They're just going to waste your money on stuff you don't want or need - I put together my own computer for $600, with more than enough stuff to record anything (hell, the thing does my physics homework faster than I can toast my breakfast bagel) and if I had taken it to a shop to assemble it for me it would have run me no more than $100.
Jeff