I ran two 7950s for a short while on my tx650. One was used and after being away from my computer for a couple weeks it wasn't working. So I doubt that it was the power supply. When it was working though there aren't any problems.
I think it would work. However, I'd get a 750w as soon as possible. Also, it's worth noting than the 300 series should be coming out soon which will give you better performance with a single card. If you can wait, I would
I'll go ahead and stick with the current PSU for a month or so, then I'll get a new one.
I'm not planing on upgrading to the r9 3XX until after it's launch to avoid paying a premium. Since I'm planning on doing that, I figured a $100 investment to slightly increase my gaming performance shouldn't hurt.
I'm actually doing a complete overhaul of the PC. Swapping in a Corsair carbide air 240, corsair h100i, corsair sp120 fans, and the second hd 7950. Later when I upgrade to the r9 3xx series, i'll upgrade my mobo and cpu with it.
EDIT: I found this video of someone doing a crisis 3 load test with two hd 7950's. He's running them on a seasonc 550w psu. This is making me wonder if 750w is actually necessary?