Quake 4 is based on the engine that powers Doom 3 and is an OpenGL game. Quake 4 is a very relevant test as this is obviously a very popular multiplayer game. Having a low framerate while competing in a local LAN is only going to hurt your chances of winning. We tested with in-game quality set to High.
The X1600 Pro is quite a bit behind the 6600GT in Quake 4 and barely holds off the 6600. Quake 4 in almost all cases will be faster on an Nvidia card.