I didn't knw you could run these two together at the same time "good enough to run a GTX 560 and a GTX 480? "
google "GTX 560 TDP" and you'll find that the 560 draws 150W. Ditto for the 480 = 250W. Assume the rest of PC runs around 150W. Your max draw is around 400W so a 500W PSU would give plenty of headroom. The card together(!) put you in the 650 and above range.
Agree with comments on good va. bad PSU. http://www.hardwaresecrets.com/article/Coolmax-CUL-750B-750-W-Power-Supply-Review/977/8
"Coolmax CUL-750B 750 W burned while we tried to pull 500 W from it at high temperatures. It burned exactly on the same point our first sample burned. The components that burned were one of the switching transistors and one of the +12 V rectifiers. Therefore, this unit shouldn’t be labeled as a 750 W unit, but as 475 W instead. Note that this unit has no protections to prevent it from burning. On next page we will show you the video from this power supply burning when delivering 500 W."