Hi - the most important thing is the brand you buy.
Stick with Seasonic, Corsair AX,HX or TX series, Antec, XFX, PC P&C, Enermax, Rosewill Capstone or Fortress.
If you can afford to, always go 1 or 2 steps up from the budget/opening price point series, ie better
components & usually (not cheaper) better value for your $$
As far as wattage goes, you only really need about 400w if you're gonna stick with one GPU, but
if you're like me you'll want extra headroom, so I'd stick with something in the 500-550w range.
Tom