Hi newcomer and welcome to the Tom's hardware forum.
1- What other components do u have? CPU, mobo, GPU, HD.
2- Do u want Crossfire or SLI?
3- Do u want OC?
Usually a 550W or better is a solid PSU for a gaming rig but be sure that's from a good manufacturer like Corsair, Antec, Silverstone, PC Power and Cooling. This Corsair 650W
is very good and have a very good future proof.
Don't buy the PSU without post ur components here for a better advice.