Failing Power supply when using SLI? or something else?

So I bought a second GTX 660 Ti a few weeks ago, and hooked everything up all fine, everything seemed to be running at least as well as it was before I added the second card. But I installed the base version of 3D Mark 11 a few hours ago, just to see how my system ranks as is right now. I'm planning on buying a bigger case with more ventilation, a Corsair water cooling loop, and a new PSU soon.

So anyway, I run the test, everything goes well. Until I notice the finished score wasn't quite what I was expecting.... My Mark 11 score was fairly low, then I noticed that it said I was only using one of the GPU's. "Ok then, guess Nvidia control panel decided to turn SLI off for some reason" I checked, sure enough SLI was disabled.

I enabled it and tried again, right after the first sequence both of my monitors went black then said they didn't have a signal. I thought that Mark 11 was just messing with the resolution so I waited a few minutes. Then I hear the "boot beep" (or whatever it's officially called) first once, then a couple more times. I felt I'd waited long enough and shut the system down. (task manager was not responding at this time either)

I tried turning SLI off again, half way through turning it off. Another crash.

SLI is now off, and the system is running like normal.


So, question is; Could this be the result of a insufficient power supply? or could there be something else going on here?
I've only built a few systems over the last few years, so I still feel like someone with more experience could think of something I'm not.


Here are my specs in case they help;
Intel Core i5 2500k (3.30GHz)
Corsair Vengeance 4GB x2 (8GB)
Asus P8Z68-V PRO/GEN 3
(2x) EVGA GTX 660Ti 2GB Superclocked (972Mhz)
Antec 25 Power supply (650watt)
Patriot Wildfire SSD (120GB)
Caviar Black hard disk drive (160GB)
Seagate Barracuda (1TB)
In Win Griffin Yellow Case
Two Asus 21" Monitors
Windows 7 HP