Just seen this thread so thought I would join in (although technically I could have done before now...)
Mmmmmm card sandwich
Finally a shot of them in my case:
After finding a single GTX680 overclocked to 1185/6800 somewhat lacking overall in Surround gaming -BF3 was fine but Crysis with CCC lvl 6 and texture mods seriously wasn't - 23fps average - I decided to treat myself to 2 Asus Direct CU II 670's.
Absolutely love them. Each one is as fast as my old 680 at "stock" clocks thanks to higher boost. They also clock to 1300(1280) / 6800 as well which will come in handy.
I will put a large review thread in the next couple of days comparing them to the single GTX680 and in some games a overclocked HD7970.
A quick snippet though:
BF3 @ 3600x1920 - Ultra settings with 2xAA
GTX680 @ 1185/6800 - 37FPS average
GTX670 SLI @ Stock - 58FPS average