if you don't mind rebooting into os9, these are games i used to play on my lombard 400mhz laptop w/ a whimpy graphics card:
*baldur's gate (rpg game, lots of fun)
*starcraft have played in x as well, works fine
*the new value line of games at
www.macplay.com looks great and they are only 20 bucks! fallout looks like the best
*civ3 might run ok, civ2 would run great!
unreal tourney, quake3 should be fine
you don't know jack has aged well (trivia game)
for osx:
airburst is a shareware game that is fun
starcraft runs fine in x
fallout should run fine as well