Check out WackyModder84's vids out, he's got a big fat list of games to play on awful computers.
It starts here: http://www.youtube.com/watch?v=Bo_XLomYKOw
You'll find a bunch of gems in there that you've never heard of. Shit, why has no one told me there is a Megaman version of Doom?
Some of my favorites from his list:
Shogun: Total War
Megaman 8-bit Deathmatch