You could get away with using a smaller power supply (ignore the advice on their website; that's far too high). The max power of the graphics card is 110W and it will actually use much less than that under full load, and I assume your CPU has a TDP under 125W. That's 235W, and those are the most power-hungry components in a PC. The rest put together won't take 50W so total 280W worst-case-scenario. You could get a 400W PSU and still have a huge margin.
EDIT: Here's a typical PC with a 4830:
http://www.techreport.com/articles.x/15752/8
Worst-case system power was 208W for everything. That was with a 130W TDP CPU, which is crazy. So you'll use much less than that. A400W PSU would be more than enough.
EDIT2: Forgot about PSU efficiency. Most are only about 80% efficient. Changed my recommendation to a 400W one, though in the typical 208W case it's still overkill. Buy a PSU with the 80%+ rated certification.
