I can't even believe I am actually thinking of doing this, buying some equipment exclusively to run F @ H! Anyways, you would need something cheap and effective.
I am thinking: an ECS motherboard (K7S5A cuz it has networking built in too), Athlon T-bird, XP, or Duron CPU (depending on price/performance and budget), SDRAM (could buy cheap second hand. Does F @ H depend on memory bandwidth very much?) case, PSU, surplus vid card (I went to an auction and most of the stuff was crap-for my personal use-but there were a bunch of old 2 & 4 MB vid cards for ~$5. Also at computer shows sometimes); HDD (optional I suppose - could you get a linux kernel that would run off a floppy and then load F @ H via floppy?) You could get away pretty cheap with something like that, even if you had to buy the CPU new.
Jeez, I can't believe I would even consider buying a computer just to fold, considering how stingy I am.
Your thoughts?
EDIT: Silly me, I just realized you would need at least a small HD to store projects and d/l WU's. Or would you...?
EDIT MkII: no need for HD: http://forum.folding-community.org/viewtopic.php?t=35