Blue Screen Errors

My systems is
AMD AthlonXP 2600+
Asus A7n8x-E Delux Mother Board
Asus nVidia Geforce 6600LE 256MB vga card
1GB RAM (2x512)
80GB Serial ATA HD
400W power supply

when I'm playing FEAR, NFS Most Wanted, FarCry, Call of Duty 2, .. after a while computer restarts and after loading the OS it shows a message 'computer is recovered from a series error'. Sometimes it shows IRQL NOT LESS OR EQUAL, PAGE FAULT IN NON PAGED AREA, WIN32K.SYS OR NV4DISP.SYS. But if I remove a RAM and play the above games nothing happens. Also when I'm installing windows xp I'm getting the error IRQL NOT LESS OR EQUAL. Again, if I remove one of my rams nothing happens. Can anyone tell me what causes above errors?

The "IRQ_NOT_LESS_OR_EQUAL" indicates a driver issue.

"PAGE_FAULT_IN_A_NONPAGED AREA" is most commonly associated with faulty hardware and can be caused by bad memory (system memory, video memory or L2 cache)