the 5750 will take up an approximately 100 watt if i remember correctly. If that's the case, 450 watt should be fine. the only problem i see with that is you're not leaving a lot of room for upgrades in the future.
The GPU and CPU match each other well enough and the mobo works to just plug all the stuff into. It would match better when the CPU is OC'd, and if its just a moderate OC, the 780g should be able to do that. If you were going for a high OC, then the mobo should be something with a 710 or 750SB.