PC P&C Silencer 500w
Bang / Buck doesnt really apply to PSU's. You get what you pay for. If you want to risk your components go cheap and get more advertised wattage.
If you want to be able to trust that you are getting what you are paying for then get a brand eg Corsair, PC P&C, Antec.
I selected the one above as its reliable, provides enough power and is quiet.
However it wont handle crossfire 4870 so if you plan to go that route then step up to the 750w.