Not a hell of a lot.
I have an AMD 550 mhz laptop with 128 MB of RAM and it is terrible for games.
Even Baldur's Gate II, which is basically a 2D game, started to chug a little after playing it for a while.
If you're not running XP or 2000 on it then you can probably get some of the old classic emulated arcade games to run on it. They are still pretty fun.
Here's some good places to look:
http://home.wish.net/~cvdelzen/Index.htm
http://www.vg-network.com/