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.