Matthew bishop2 :
600 bucks and I got a 500W psu
Go for a gtx 760 if nvidia centric.
If amd is on the table go for a r9 270x.
Anything higher would bottleneck, and you would also be looking at a cpu upgrade.
They should run fine on a proper 500w, if on a dodgy quality psu get something like a xfx 550w to go along.