Call Of Duty 2
The Nvidia GeForce 8800 GTS performs horribly at x1 and x4 speeds, quite far away from its maximum performance, which is clearly reached at x16 speeds. The ATI Radeon X1900 XTX, however, doesn't require more than x4 PCI Express bandwidth to perform properly in Call of Duty 2.
The situation is different in Quake 4. Here, both the ATI Radeon X1900 XTX and the Nvidia GeForce 8800 GTS perform adequately at x4 PCI Express speeds, and gain only a little by running at x8 or x16.
The 3D graphics benchmark 3DMark06 by Futuremark is very GPU intensive, which means that it was designed to stress your graphics processor and doesn't require a lot of interface bandwidth. Nvidia's GeForce 8800 GTS suffers more from inadequate PCI Express linking, while the ATI Radeon X1900 XTX already performs close to maximum at only x4 PCI Express speeds.