I plan to upgrade my computer (250W + 6150SE/AMD Athlon X2 5200+) with XFX 5770 GPU unit. Then later, when I save enough money, I'll take away the 5770 and put it in a new computer with Phenom X4 965. But right now, I only have money for the GPU + the GPU. So short question, what PSU would be able to fufill the duty of a XFX 5770 and a Phenom X4 965.
For a basic system (as I don't know what else you will have in it), the minimum wattage is 260W, with a recommended wattage of 310W.
Therefore, you should get at least a 350W, but might want 400-500W, depending on what you forsee in the future.
If you crossfire, it goes up to 370W recommended. So the 400-500W might be the sweet spot, leaning towards the 500W, just for room, and safety. But also, make sure you get a 80+ cert psu from a reputable manufacturer, and you should be all set.
^^ In short, get a quality 400-500W(If no plans for Crossfire) or 600-650W(If Crossfire) from Antec EA or TP or ECO series/ CoolerMaster Silent PRO or Real Power/ Corsair/ Silverstone/ Seasonic/ PCP&S/ OCZ Fatality or Mod Series/ Enermax