Brands matter a lot in PSUs. Seasonic is the best currently, and they also make PSU for corsair, xfx, and antec among others... Graphic card manufacturers often spec their card requires a bigger PSU than it does just as a safety threshold. As above poster mentioned, the amperage is a critical factor.
Bottom line is you should buy a reputable brand PSU. 650 watts is quite safe for a single graphics card solution, 850 watts is safe for a sli/crossfire setup. Quite often you can get by with less on both accounts.