I just bought a cheap Acer computer for $480. It has an i5 3330, 6GB RAM, 1 TB HDD, and a GT 630 2GB. I want to upgrade the GPU. If I get an AMD 7790, but keep everything else the same, what power supply should I get?
Your 7790 will need a 430w psu with one 6 pin connector.
Newegg has a Corsair CX430 on sale for $20 after rebate.
Not a bad psu. But, I think I would go stronger to allow you to upgrade the graphics card in the future. 500w or more perhaps.
Look for seasonic, corsair, antec, xfx, pc p&c for quality units; do not buy a low quality psu.
If you buy a Corsair CX, make SURE it's one of the "M" models, one of the modular ones. Although they are all built by CWT, the non-modular ones use some inferior Samxon capacitors that don't like heat and are known for early failure. Fine in light-duty office builds, I wouldn't put on in a gamer.
The XFX linked by SR-71 would be a nice choice. Others to consider are the 450W Rosewill Capstone and the 380W Antec Earthwatts. All would do.
Here is one list of psu's sorted by quality tiers. http://www.eggxpert.com/forums/thread/323050.aspx
It is worth while to spend a bit more on a quality unit.
Also, buying a 20% stronger psu than you need will allow the unti to run more efficiently and be quieter. It gives you some leeway for a future graphics card change.
While not bad in theory, in practice that list is ancient and therefor obsolete.
A much more current, maintained listing may be found at http://www.johnnylucky.org/power-supplies/psu-recommend...
He cites competent technical reviews to back up his recommendations as well. Similarly, he's got a Lemon List there too, indicating [some of] the junk to avoid.