Why my pc gets hang while play game on it ?

my specification is
intel core 2 duo e7300
2gb ram, 320gb hard disk,
nvidia gt 220 1gb graphic card ddr3

i have purchased the graphic card approx. 2 months ago , it works the whole day before but now pc gets hang after one hour, is it due to heat or i have the power supply of 350watts, that power supply is creating the problem, how i can check whats the problem, if power supply is creating i have not good knowledge about computer , i have listen with some body we can share our ram with graphic cards , if it is the problem due to that then how i can solve it, is underclockig is a solution , how we can do that.


pl. solve some body my problem i will be thankful to him.
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