Which graphics card to use....?

Hi

I am quite new to hardware aspects of computer. Recently i built a new one and the configuration is as below:

Processor: Intel Quad Core Q9550 (2.83 GHz 1.3 GHz FSB)
MoBo: DP45SG
Memory: 2GB DDR3 (Not sure about the speed)
HDD: 500 GB SATA

I am not a regular gamer and use the comp mostly for watching movies and rarely for gaming. I use it quite often for programming as well.

I am using a 22" monitor at resolution of 1680X1050.

Earlier I have worked on some machines which had no additional graphics cards. (MoBo: Intel 845 or something of that kind). I had no idea that the DP45SG doesnt have onboard graphics support. So I didn't worry much about the graphics card. But I came to know of the fact at the last moment and in a hurry i bought a GeForce 8400 (512 MB) Graphics card.

Now the system hangs when i try to watch any movie. It works well for some time. But after an hour or so it hangs.

I think that the graphics card is heating up and not performing as expected.

I have done all necessary driver updates (MoBo Drivers, Graphics card Drivers).

Please suggest me on what i may try next.

Thanks in advance

What PSU, OS are you using?

Thanks for the fast reply. But as I said the GPU works fine for some time and then crashes. Is it possible that it is wrongly configured....?