I was thinking about MAME, but when I was into downloading roms about a decade ago, MAME websites where confusing as hell to navigate, disorganizing, annoying with 'vote for me adds', and just plain a hastle to deal with. Plus, most had errors in them.
If Everything where available on XBox Live, or Wii Virtual Console, I'd go that route. But for now, I'd rather find as many boards as possible. Only ones I need now, assuming the old ones are still working, are Pac-Man, Mrs.Pac Man, Popeye, Star Wars, ROTJ, and X-Men. I was toying around with getting NBA JAM ( I think that was called) and Lethal Enforcer, but I dunno if Light Guns are sold.. These are the only games I'd love to play regularly. Anything else is just extra.