if you dont mind me asking, what psu do you have? im thinking of going SLI with another 7900gt ko and i currently have a 500w psu. I think i should upgrade to a bigger psu before setting it up.
as for what games, i would try Fear. Fear is one of the most grapically demanding games out right now. its also nice to run 3dmark to get a sense of how well your system is running compared to others with similar setups.