Would 2 GTX 660 perform just as well or better than a GTX 690, is it worth the money? or should I stick with one graphics card, would a 750w power supply suffice for 2 oh these?
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No, 2 660s will not outperform a 690(2 gtx 680's). A good 750w psu willl power 2 660's, while a crappy one will probably just pop and fry the system. Is it worth it? Well that depends on what games, resolution and settings you want to run.
Two 660s will perform better than a single GTX 680. As bignastyid stated, a 690 actually has two 680 gpus on a single board.
I use a Cooler Master GX - 750W 80 PLUS Bronze Power Supply (RS750-ACAAD3-US) <-- will this be able to power 2 GTX 660?, also even though Im disappointed my graphics card won't be as good as a 690 its still worth it to get another 660 in a couple months and be equal to a 680.