Racing games are always fun. Re-Volt could work quite well with a joystick, and Need For Speed 2 & 3 too. I've played them all in a P3-600. This Stunt GP reminds me a lot of Re-Volt, but I guess it'll be as fun. Killer Loop is something in the middle of Re-Volt and Wipeout, also quite fun racing game.
For non-racing games, I like Rocks'n'Diamonds, a freeware remake of Boulder Dash and similar games, with lots of levels. It's at
http://www.artsoft.org/rocksndiamonds/. I also played quite a lot a tetris clone called Tetris Worlds.