As someone with a GTX 480, buy it! It's a great card but runs really really hot - you're going to want to put some VRM heatsinks on the ram if you choose to get a universal gpu cooler. Personally, I would opt for a full watercooling block as it really does need it. Having said that, I'm running a 480 with a aio watercooler on and vrm heatsinks and its just fine - for overclocking, get a full water block.
Don't skimp on your case - getting a generic one might turn out to be a complete waste of money.
Hope this helps.
As for the motherboard, it depends on what you want to do with your hardware. If you want to overlock the hell out of everything, you might want to consider getting a motherboard capable of handling the overlock.
You don't need more than 620W - the cpu doesn't draw that much power and neither does the gpu. I'm running a similar set up on a 550W psu and it works without a hitch.