Another ...which graphics cards should I get question

I have PCI express MOBO running an AMD Opteron dual core (185) with a 650 watt corsair power supply and 4 gb of ram....I have two NVIDIA 8800GTS running in SLI but I can't play any game that requires intense graphics like starcraft II, serious sam, crysis....the list goes on and on. The computer keeps crashing. I updated the graphics drivers and everything else ...I checked the video fans, dusted the case (the case is air cooled but it's suppose to be real good ...it's a large silverstone. ....the only way I can play my games is if I remove the cover to the case and use a 12 inch Blizzard air fan to blow directly on the cards ...then i can play any game and the games stop crashing ...I am tired of it .....I dont' want two cards running SLI anymore .........the cards are blazing hot and are probably frying my other components ........I just want one good card that won't overheat ...that I don't have to use an external fan on and that is around 200 dollars ...give or take ...please help ..thanx.

btw I think NVIDIA really blew it with these 8800GTS cards .....they are huge cards that barely fit in the case and require huge passive cooling mounts as well as fans .....sure they provide good graphics .....if you can prevent them from MELTING......when I bought my cards back in the day I looked at benchmarks ....I should have looked at ergonomics and things like temperature instead.