it depends on what GPU you are going to put in it do not buy cheap PSU's they are more trouble than they are worth generally a good 500watt is good but if you are going to buy an AMD card then you might need some more wattage
PSU Tier list tier 1-2 are good for gaming
http://www.tomshardware.com/forum/id-2547993/psu-tier-list.html
I would recommend an EVGA SuperNova Gold G2 / or new G3 /GS / GQ, Platinum P2 / PS, or Titanium T2 or a Seasonic M12-II Evo or S12-II
GOOD PSU EVGA is a B2, G2, G3, P2, GS, PS, or T2. Avoid the EVGA B1 and G1 series/ I bought a EVGA G3 with 7year garentee