Dude, this is what i tell everyone - don't settle for anything less than 8800 GTS - TRUST ME - wait a little bit & save some more money and buy GTS, YOU WON'T REGRET IT!
If you need to, buy one card then wait a while then buy another card for Sli - its what i did
If you're sure you want Sli you should play it safe and get a good 800 Watt+ PSU