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.
So far, I've got the aforementioned motherboard, two Kingston hyperx 4gb ddr3 ram sticks, an AMD Pehom II X3 720 black, and a used videocard that I believe is a HIS Radeon 550 512mb card. The sticker was kind of hard to read, seeing as it was used. This is the one part that I can replace without too much hardship, due to the fact that it was only 30 dollars.
I am planning on buying a Western Digital Caviar 500 Black Hard drive and a Samsung SH-S223F optical drive soon.
So, does that help?