Quake 4 and Doom 3 are based on essentially the same game engine, so as you'd expect, the performance landscape in our custom Quake 4 benchmark looks much like Doom 3's. In this test though, the Radeon X1800 XT has a measurable advantage over a 256MB GeForce 7800 GTX when anti-aliasing and anisotropic filtering are used. Without additional pixel processing though, the Radeon doesn't look as strong and gets beat by significant margins. Factor in the performance of the new 512MB GeForce 7800 GTX, and NVIDIA's flagship distances itself from any other single card configuration.
Looking at the performance of the SLI rigs, we see a pair of 512MB GeForce 7800 GTX cards easily outpaces a pair of 256MB GeForce 7800 GTX cards. The 512MB version's much higher core and memory clock speeds, and larger frame buffers allow it to jump way ahead of the pack. In fact, the 512MB SLI rig is over 20 frames per second faster then the dual-256MB card SLI rig at 1600x1200 when AA and aniso are enabled.