Comp locks up with geforce gts 450

Ive newly built a computer and it was my first time. everyhing seems to work fone except for when i play games. My computer locks up when i play games that rrquire 3d. Is sometjing wrong with my graphics card? Ive already tried unstAlling and reinstalling the drivers from nvidia and problem still persists
My psu has 1 pci e line with a 6pin as well as a 8pin pci e. When i use the 6 pin end, comp freezes pn welcome screen And when i use the 8 pin side (omitting the extra 2 pins) i caj actually run ok. Locking up during games are inevitable though. Any help is greatly appreciated.

Psu - antec neo eco 620w
Gpu - galaxy geforce gts 450 factory overclocked
Mobo - biostar a770e3
Ram - corsair xms3 1333 ddr3 (4gb)