If you have that opportunity to try a PSU and return, I would definitely suggest doing so. My friend had a "generic" 600w psu that came with his iBuyPower PC. It could not handle a 6950 at all. It would stutter, frames would drop constantly etc. We bought the exact same gpu, it was a sapphire 6950 unlocked to 6970. I had no issues. But he had so many issues he swore he'd never buy a sapphire card again. Because he was so pissed he ended up buying a $140 850w Gold PC Power and Cooling PSU lol. He has not complained since .
You dont have to spend $90 on a psu. Also you dont need more than 600w even if you are doing crossfire or sli. A good solid 90% efficiency 600w is enough.