whats the least hardware that it'd still be worth running it on?
Are you looking for low price or minimal performance. The Efikas been mentioned, but I've seen 1.25Ghz Emacs running at about $35. If your willing to put up with 2D graphics only the 1.42 Ghz eMac will also work (and has 64mb of video memory).
Powermacs are also cheap. I have one I paid less than $50 for. Anything 733Mhz or faster is fine (you could even consider slower models but why).
Powermacs also have several PCI expansion slots.
