PCIe vs. AGP version of the G550 in vertex throughput. Showing FPS ratio of the PCIe to the AGP on Y, triangle count on X. Rendering a static textured mesh using Direct3D 7 in Windows 98.
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So the PCIe version starts off at 80% of the speed of the AGP version, makes sense since it's clocked at 80%. But with anything more than 500 triangles it takes a massive extra hit. I can't imagine the PCI bus being this gimped vs. AGP so it's more likely the bridge chip, the TI XIO2000. But then I don't know.
In this particular test the PCIe version tops out at about 300000 triangles per second, the AGP version does just over a million.
So the PCIe version should do relatively well in low poly games, and suffer more in games with poor visibility culling. But from what I've seen the polycount probably isn't the only factor.