Both the old C2D E6500 and the new P2D E6500 are definitely enough for the GTX260. I've seen people pair it with E5200s OCed to 3.0GHz (basically the same as a E6500), and you can OC yours to 4.0GHz easily on air.
I personally have a 4870 (equal to the GTX260, but more forgiving on less powerful cpus due to drivers) and it works fine with my E5300 with a small overclock.
As the "building a balanced PC" article by tomshardware showed, you need at the very least, a fast dual core w/ 6mb of cache (ie. E8400, preferably overclocked) or preferably a quad core for Nvidia SLied cards. (ATI Xfire is more forgiving due to different driver types)
A GTX295 (performs equal to two GTX260s) paired with a E6300 actually scored lower than a E6300 with a single GTX260 since the CPU couldn't power the graphics card effectively.