500mhz is fine, as long as you get the right emulator. Go find the VirtuaNES homepage, and download the english version, that one runs fullspeed on my 333mhz celery, just make sure to get 256mb of sdram at least! Ram is a pretty big deciding factor whether an emulator wants to run smooth on your computer or not. Although the faster your processor gets, the less ram you gotta worry about. Also, if you're gonna have an NES emulator in there, might as well put a sega master system emulator in there as well, since it has less buttons I think *2 per joystick, 2 on the console* and would be very fun for your old school needs. Get MEKA for that one, very good emulator, although you gotta register to get the newest versions *paying money for an emulator, eww*
You can get a 500mhz machine with 256mb of ram on Ebay easily for under 100 bucks, and if you got a computer laying around, you could probably add in a new motherboard with at least 256mb of ram with like a 1ghz processor for under a hundred.
-Remember to take pictures along the way, so us BYOAC guys can see
-Luke