I have problem playing games?

i have bought new computer at Kuwait it is a hp company and it has 8gbram and gt230 (1.5gb) VGA card now when i play a game after some time it just stops working the computer is running but the monitor show no signal and i bought new monitor it is samsung 21 (1920*1080) resolution now when i play it just stop as before and monitor is like when it is on and the cpu is off i do not know what to do change its motherboard or vga card and my power supplier is 300 watt