Corsair 450VX at the minimum. If your system is more loaded (quad CPU, overclocking, lots of disks, etc.) then 550VX or 650TX. Also PC Power & Cooling 610W.
Is your MB a SLI and you plan to add a second card later? Then get a Corsair 750TX or PC Power & Cooling 750W.
is my 500W okay with it? its Thermaltake 500W PSU