|"My current power supply is 680W with two 12V rails at 43A (21A on one, 22A on another)"|
Adding the +12v amperages together is NOT how it works. Look at the label on the side of the unit & see if it rates the combined +12v rails. For instance, the following Antec label shows three +12v rails at 18A each for a combined wattage of 504W:
The total amperage is NOT 3 x 18A = 54A, it's 504W / 12v = 42A
Regardless, 680W isn't a popular size & a quick look at newegg shows only garbage units by Apevia, Okia & Sunbeam. I wouldn't trust any one of them on a high end system. The GTX 480 sells for about $400 so you should have a decent quaily power supply to protect your investment. And hopefully you have a top of the line CPU (Core i7) & lots of RAM or the GTX 480 is just going to be wasted on your system.