I don’t understand your question about the 680 or 670. But you cannot SLI a 680 with a 670. Also, an overclocked 670 is virtually as good as a factory 680. Get two overclocked 670s, same model and same manufacturer.
I’m not sure your motherboard supports three-way SLI. Please note that “Quad SLI” is not the same as four-way SLI. Double check your motherboard’s specs, because if you ever want to do three-way SLI, you might not be able to.
I don’t know a whole lot about power supplies, but I like to play it safe and get more power than I need. I have a thousand watts for my dual 670s, but I probably don’t need all that power. But I like to be able to upgrade graphics cards in the future without also upgrading my PSU. Just be sure to by a good brand. Everybody seems to love Corsair.
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i7 3770K OC @ 4.2Ghz / ASRock Extreme 4 Z77 / evga FTW 670 2X-SLI / Creative SB Audigy SE PCI Sound Card / 32 GB Corsair Vengeance 1600 / 128 GB OCZ Vertex 4 / 2 TB WD Caviar Black / 2 TB Seagate Barracuda / Asetek 510LC Liquid Cooling System / LG 14X Blu-ray R/W / Corsair 1000w PS / AzzA Genesis 9000 full tower / Windows 7 Professional 64 / 144hz 1ms 24-inch Asus monitor