If you want it to be usable under Panther (10.3) or Tiger (10.4), I wouldn't buy anything less than a slot loading with a processor in the 400-500MHz range. I use my iMac G3 "Blue Dalmatian" (500MHz) for some light tasks under Tiger, but if that is what you get, expect quite a bit of lag when it comes to the internet. Also, if you choose to run Tiger, you should probably disable Spotlight and Dashboard with a program like IceClean.