Let's just say for instance that I have full collections of ROM's for NES, SMS, Genesis, SNES, GBA, N64 and PSX on my PC. What would be the best USB controller to use for them? Or would I be looking at buying a few different ones?

Where would be the best place to look for them be, other than eBay?

Cheers,
Omecle

Edit: In fact, scratch PSX, I can convert them to EBOOT's and play them on my PSP.