When I first got my imac (also with a 256mb x1600) i remember that it ran WoW pretty well with almost all settings turned up (including full screen glow). I haven't played that in a while though.
It runs team fortress 2 and half life 2 well at medium settings. TF2 actually looks really nice on it.
EDIT: i'm running on on a first gen iMac, so my processor is slower than yours. Are you using an iMac or an MBP? if you're using a macbook pro, then the card might be underclocked more than mine is...