What res are you playing at and what settings. It REALLY is a depanding game. AA and AF really put a hurting on it as well. Try turning off HBAO it can really take down your FPS. I borrowed an 8800GTS/512 while my 5870 is in for RMA repair. I am running on an i7 @4GHz at 1680x1050 and my details are on high, AA & AF are x1, and HBAO is OFF. I get about 60FPS most of the time, VSync is ON. So with a GTX260 you should be able to turn up the AA and AF to x2 or x4 maybe.
Here is a little slideshow of what HBAO is. It is basically a lighting effect.http://www.nvidia.co.jp/object/siggraph-2008-HBAO.html