The performance of top-end graphics cards is largely limited by the speed of the CPU in this game. That’s why there’s little difference between the cards. Yet it is clear the Sapphire Toxic X1900 XTX is competing with the GeForce 7900 GTX quite successfully, although the latter plays on home turf: the Quake 4 engine uses the OpenGL API and the stencil shadow technique. The average performance of the cards is never lower than 114fps in 1600x1200 with enabled full-screen antialiasing and anisotropic filtering.
Serious Sam 2
The GeForce 7900 GTX seems to allow using the resolution of 1280x1024, but you may want to switch to 1024x768 to have some speed reserve and avoid slowdowns to below 15fps. But the Sapphire Toxic X1900 XTX and the Radeon X1900 XTX offer a bigger speed reserve in 1024x768 than the GeForce 7900 GTX does.