975X chipsets run the older PCI-Express 1.0/1.1 standard, so could be the card might be exceeding the slots bandwidth and holding it back there?
C2D E6600 OC'd to 3.2GHz should be fine as far as I know.
your not going to notice much in games jumping on to a Quad core, from your current CPU, since there's few if any games that use 4 cores right now.
I'm willing to bet its the PCIE 1.1 slot holding your card back. since many have considered a Radeon 4870/Geforce GTX260 on the borderline of peaking out PCIE 1.1. seeing as the 275 is way past that, its almost a given.