The PSU is borderline.. it may be fine since you are not pushing a quad core however since you are clocking that PSU is really what I would call on the fence for taking on a 280 series card
You should have a true 650 in the system for that card, 750 if the processor was a quad
those 'rail' PSUs can drop power quckly which is why I only buy single rail PSU's which deliver the current to any plug on the rail without loss
PC Power and Cooling is designed that way... and they have the best 750 watt PSU desingned on the market for less than 120 bucks
http://www.newegg.com/Product/Product.a ... 681770300980 bucks with the rebate.. that PSU will follow you for years