As long as you're not overclocking the cards much you should be fine. I was getting random shut-downs with the parts in my sig (cards were overclocked) on a quality 800W PSU. That happened playing Starcraft II and sometimes running through the STALKER: COP benchmark. My Kill-O-Watt indicated 770W at the wallplate. A new 1000W PSU solved the issue.
firstname.lastname@example.orgGhz | Maximus VI Hero | 2x290@1150/1450 | 16GB DDR3 | Custom H20