I came across your post as I'm having the exact same issue as you. I have the same MB and dual Gigabyte GTX460's. Only difference is this issue started for me when I added an add'l 6GB of ram to my system for a total of 12gb. Once I removed the add'l ram or one of the vid cards, everything works fine.
Made a post before I found yours here: http://www.tomshardware.com/forum/288349-30-added-work
Was wondering if you resolved this issue, and if so, what was the resolution?