At that resolution that would be the most you can expect, and I must say that your expectations are quite high from a X1900XT.
Oh well see on the mac side using openGL i get about 50 or more fps on all high settings in the half life series, killing floor, counter strike source and so on. i've never had a steam mac game that it doesn't blow away. But then again directx is a whole another monster and i do mean monster. it's not nearly as nice as OpenGL