You can SLI the 6600 but,,,,,, 2 6600 dont even equal to a 6800 .. I wouldnt recommend dat card because your games will run in slow motion, the only 6th gen cards able to handle todays games are 6800 ultra or x800 equivlant ,,
That 6600 is as fast as a geforce 5900 ,, its a budget card, and putting together two budget cards equals,, budget performance,
Do yourself a favor and buy a 6800 my friend,