Handling the memory will be the bigger part of that equation being Intel based.
Any way you go you'll need 1 gig of ram minimum. If you go with the 64bit version of XP as I will be doing by this summer the minimum will be 2 gig or ram. My plan is to start with a pair of 2gig ram slips and eventually taking it to 8gigs of total ram on a dedicated audio workstation.
I prefer AMD's imbedded dual memory controllers in it's Athlon64 & X2's as they will have fewer bottleneck issues when the 64 bit software starts becoming available for XPpro's 64bit version.(E-mu already has 64bit versions of Proteus X2
& Emulator X
as do Steinburg's Cubase4
Also make sure your PCI Audio card\interface has it's own DSP for optimum performance.
And be sure if you do plan to go towards 64bit OS & Apps, and record in 24bit, you will need BIG and Fast hard disks. I'm starting with 500gb and expanding it to 1tb in the next year. I like having multiple projects going at the same time and don't want to have to migrate projects on and off a working surface. I'm now investigating partitioning each drive into 5x100gb segments so each project can stay in one partition as not to allow fragmentation of multiple projects across one large 500gb partition. That way, if you have more than 10 projects at one time, you just add another 500gb harddrive as required.
Pretty soon, 2 tb of storage won't be out of the question if your a prolific writer.