Reboot Problem with i7+XFX GTX260 Black Build

Guys,there is a weird problem with my i7 system,I'm using a XFX GTX260 c216 black edition now which was bought along with my entire system parts last week,problem is whenever I play games like far cry 2,just few seconds into the game,my system would just reboot,while playing less intensive mmorpg games or watching a movie is fine,is this more likely the psu problem or gpu problem,my power supply is a Hec 750W,i know it's a off-brand,and the +12V rail is 19A while my gtx260 require 40A.I'm not sure which one actually casuing the reboot since all parts are new arrival prior to my build for less than a week,please enlighten me,thanks alot!
Rig Specs:
i7 920/AsuP6T Deluxe/6GB DDr3 Corsair/WD 640GB/Hec 750W/XFX GTX260 C216 Black