SLI 8800GTS 512 and 1280x1024? You are already at way overkill.
A single 8800GTS 512 will run nearly everything you throw at it quite handily at that resolution. You also know that that 8800 overclocks very well? Keep the shader clock and core locked, raise the core speed to around 775. (stock is 650)
Then raise the memory very slightly, a little at a time till you start having errors.
The memory is pretty sensitive, usually around 1100 is about the best you will get.
I usually just leave the memory a stock, and work with the core/shader clock.
To do any better than what you have, you would need a GTX295, or a 4870X2.
However, once again, at the res you are running, you don't need more than 1 of the cards you have right now.