It's weird.... w SLI when something explodes the FPS goes to 3 lol....
That means cpu bottlenecking. Crysis is heavy on physics. Those pieces of debris all need cpu to calculate trajectory. Either overclock your cpu, or you can turn physics details off, which will affect gameplay, of course.
20-27 fps is still low for dual 8800ultra. The crushes may be due to psu not supplying enough power, or overheating.