Conclusion: X4 PCIe Is Usually Sufficient
The professional benchmark SPECviewperf 8.0 is generally quite susceptible to changes in available graphics bandwidth. In contrast, we found that games, as we expected, have much lower demands.
The applications we used (e.g. Doom 3 or Unreal Tournament 2003) require a minimum bandwidth of about x4 PCI Express. Above that setting, it was almost impossible to feel the performance difference. This is especially surprising considering that the rather challenging 3DMark 05, which can be described as a technical trend-setter, is not exactly conservative in terms of graphics features.
The result is even more interesting in light of the anticipated nForce4 SLI chipset, which will be available by Christmas time. This beauty either operates one PCIe graphics card in x16 mode or two similar ones (same chip and memory) at x8 PCIe each. Even though it isn't possible to double the 3D performance by using two graphic cards, there is no doubt that PCI Express will not be a bottleneck in this arrangement, not even if it is set up to operate at a slower speed such as x8 PCIe.