Your PSU will be okay, the cards won't use that much power collectively. The cards won't draw too much more than 150 watts each under load.
So even pretending the cpu is 150 watts , 2x gpus at 150 watts under full load your still fine
Although 650 is the lowest I would recommend 700 watts will be great. Hopefully it's modular // has the pins to power them.
However you may have some microstuttering going on cross firing a weaker card like that.
You didn't ask for it, but I'd suggest 6950's if it's not too late
source: my brain and passion mixed with a link so I look legit.http://www.legitreviews.com/article/1445/17/