First off, I'd suggest an AMD64b platform. The on-die memory controller is apparently greatly helping with latency.
For the motherboard, I'd get one with an nforce3 250 chipset. Personally, I've got an MSI Neo2 Platinum 939 based board preordered. They have a 754 version as well.
For audio, if you're going to only record music (ie, no games!) then I'd suggest either the Terratec DMX 6fire or one of the new Emu cards. Don't believe the people who say that you can't record with an Audigy, they have unbalanced inputs and their ASIO drivers are fairly limiting, but it will do if you use your system for other things!
As far as memory, it really all depends on how many tracks you expect to use at a time- same with processor speed. Personally, I run 2Gigs pf PC3700, 4x512. I would say that getting up to 1Gig is where my sweet spot was, but I use Cakewalk Sonar 3
and have about 8 tracks going at a time, on average.
Also, you need to make sure that you have a couple of hard drives, one for your OS/programs, and another for your data. Remember, the hard drive will be the slowest device in your system, so you want to make sure that you do all that you can to motivate it to run a bit faster!
Hope this helps, let me know if you have any questions about what I said!