I've just built a system with a i7 3960x processor. I also have set up to dual boot Win 7 64 and XP-64.
After running benchmarks in both OS's, I've noticed that the computer runs quite a bit slower with XP-64.
Because XP cant take advantage of the new openGL and DX standards that greatly improve the speed of rendering. Or any advantages that 7 has with multi-threaded performance. XP is a dead OS so unless you specifically need to use a program only written for XP that will not for any reason work on 7.
There is no benefit to using XP anymore and people who say there is need to go and relearn what they think is better than 7.
I use rendering software that (usually) runs more smoothly in XP-64. I know it's a nearly dead OS, but it still works better in some ways than Vista or Win 7.
As for the OpenGL or DX idea, that might account for the lower score in the OpenGL Standard test, but that doesn't effect rendering CPU test scores. For those, you could have a $2000 professional card, $500 gaming card, or $30 super low end card in that PCIe slot and the Rendering CPU score would be roughly the same each time. ( I know, I've tried it). Rendering in this case is purely handled by the CPU.