650W Ultra what? Lol.
Some generic (like some "Ultras" I've heard) have low 12v amps. 650W is plenty for 1, however 2 HD7850s on a generic could be different. A quality 650w is more than likely capable.
What make/model is your PSU, OP?
+1 TX750, too good of a deal to pass up. I've been using the V1 for a few years now without a hiccup (got it power my old 4890s back when).