I acquired a GTX 670 from a friend for teting, and it runs perfectly fine. Both GPUs get to 75 deg C tops at the standard clocks gigabyte set...
I might also add, this is totally unnecessary for 1920x1080 resolution... unless you want to play the Witcher 2 on Ultra gfx...
I'm VERY happy with a single GTX 670 from gigabyte.... best idea ever to rid of my 5970 super oven and get the much cooler, efficient fermi