I upgraded from my x1900xtx to a 8800GTS G92. The performance increase in COD4 was at least 30fps +, Supreme Commander 30fps +, TF2 30fps +, CsDOD 60fps +.
I was extremely happy with the performance upgrade.
My system:
Koolance Watercooled PC650 System
Amd 4800+ o/c to 2.6ghz
x1900xtx...