Is your Quad running at stock speed? If so, you should really try overclocking it 3.0ghz first. At 2.4ghz, and running SLI already with a couple of 8800GTS's, your bottleneck is the processor. The GPU's are dated, but I think unless you crank that quad up a little, you are still going to be disappointed with a new video card.
you already have pretty goo GPU settup. 2X 8800gts can let your run ANY game ATM. you have a 750i, so just take a 260 or 280 (or new 270 that is comming soon) and another one latter when price goes down . and YEAH jit is right, an OC would help ALOT