Yeah I had the same problem, I had only a single +12v rail as you, and that was trying to power everything with mobo/all the case fans and other things in found in there + the power consuming 88000 GTS.
Basically this produced rails to drop amps down causing the whole system to crash while playing.
But then again it depends on what other parts you have in the setup as well. Check out this one:http://www.extreme.outervision.com/psucalculatorlite.jsp
Helped me to get a recommended total W estimate.