Two 8800GTS 640mb would be faster. Although i would never advise it cause you could just get an 8800GTX and with less hassle and no need for a high end psu you can simply upgrade to the next generation of ATi's or nV's graphic cards which will be faster and much cheaper than the the 2x8800GTS 640mb setup that you would have. With less money invested initially you will be able to upgrade to better graphic card later.
This is probably the best I've seen yet as to comparing cards. unfortunately they didn't list the lesser cards in sli. But having said that, use your imagination a little.
This is probably the best I've seen yet as to comparing cards. unfortunately they didn't list the lesser cards in sli. But having said that, use your imagination a little.