1Tb is a safe bet I think. I have a 120GB hard drive right now with only MAME, ALL Nintendo except 64, all Catrtidge based sega, all atari, and colecovision and it's pretty close to full, and I think that's v120. If you want to add N64 it's HUGE, plus the laserdisc games, any CD based systems (PSX, Dreamcast, Saturn), etc. I think 500MB should hold pretty much everything right now, but it may not be enough going forward, and being that you can get 1tb drives for like 89.99 (check slickdeals/digitaldeals daily) it's probably just worth going the Terabyte route, storage is friggin cheap right now.