I can play other games at this res without a problem, as well as the Finder. This is really weird. I had this problem when I had a GF3 and the same monitor. When I installed and first launched the game, it would start in 640x480 and start to play, but if I changed the res just once, 640x480 would forever disappear as an option.

Same thing now, after a fresh install and the same setup (though with a 9000 now).
17" ADC CRT by the way.