My Alienware Laptop crashes every time i play a full screen game for thirty minuts

I know every one else says this but, im new here and new [in a maner of speaking] to computers. i have a 2006 alienware laptop and i know of a few problems i may have. it runs hot. i have speedfan and it says my comp runs at [when idle] 68*C. and also about once a week i get a messege that my virtural memory is low. i also have only 512mb of RAM. my comp never crashes or even lags when i dont have a full screen game playing, and even when i turn on a full screen game it plays perfect. but, after about 30mins of playing said game the screen goes eather white or black [depending on the game] and the game never comes back. it just stays that way untill i force it to turn off by holding the power butten. and i recently reformated my comp and cleaned all files off because i thought this problem was a software problem [i had a professional do it for me]. thanks anyone who has an idea of what i can do to fix this.