Problem with fatal error and not enough memory

hi my computer is a dell 420xps Intel core 2 duo e6850 3.0ghz 3gb memory 1.5t hard drive Nvidia Geforce 250 512mb 500watt PS network manager killer xeno pro im having the problem with black ops when im playing the game about 20min into it i get a fatal error pop up with black screen and every once in a whyii i get a pop up that say not enough memory and both i have to press ctrl-alt-del or restart computerthe games i have are as follows Black Ops,Medal of Honer,Modern War 2,Homefront,Battlefeild 2,Sniper Ghost,Combat Arms,Need For Speed,Damnation,Gun,and Zombia drivers do i have enough memory and is my video card good enough can someone help me
 
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