Will the gts 450 work on the optiplex 755?

I own an optiplex 755 minitower with e8300 core 2 duo and 4gig ddr2 ram and want to upgrade to play newest game on at least low settings with smooth framerates(crysis2-3 protoype 2 skyrim etc). I think the gt 640 will fit and has good price online but i found the gts 450. I need the one without 6 pin power connector because my psu doesnt have one.12a+12vb =combined 264w i think it said. With 18a on each i remeber getting 23amps or something whwn i calculated it. I cant find the requirments for the zotac eco or the sparkle version without external power.