Have you had your eyes on a computer upgrade with Intel's forthcoming X38 Chipset? Over the course of the next week we will be looking at Intel's new X38 Chipset under Linux and Solaris as we look at the ASUS P5E3 Deluxe and the Gigabyte X38-DQ6 (this motherboard isn't even being announced until the 10th). Among the Intel X38 features include PCI Express 2.0, dual PCI Express x16 slots, Intel Core 2 Quad 1333MHz 45nm CPU support, and removing the overclocking protection from the motherboard.
I would not expect any speed changes with one gfx card only, but as you never checked a special OC setting, how about testing this:
Usually a cpu with fsb 266/333 could run with 400 too (with DDR2-800), so just lower the CPU Clock Ratio and raise CPU HOST Frequency (FSB). It would not even be real OC, when you run a CPU with 6*400 instead of 9*266 (E/Q6600).
The funny thing with my Gigabyte GA-G33-DS3R is, that with Linux the CPU freqency shown is 9*400 then, not the measured CPU frequency. It must read out the max clock ratio and use that, so you could easyly "fake" a massive OC