RAM is still inexpensive so load up on RAM, since for speed you want as much as posible to run in RAM. I've noticed that linux kernels previous to 2.4(I've never used the 2.4 kernels) can get very greedy with memory(and swap). Also make sure you get SCSI disk drives. In my experience more drives == better speed, since you can seperate which things reside on which drives.

I think that these will allow for an inexpensive yet speedy multi-user *NIX system.

--xPhase