system crashes and restarts

[stb1swat]

I just built a brand new system not two months ago. I've recently begun to see system crashes. I get a low buzz in my head phones (what ever sound that was being played repeated) and a straight black screen then boot up. I first thought my MSI8800gts was overheating, but I soon put that to rest since the crashes are erratic and my card is never really hot.
These crashes are only in games.

Specs are
WinXP home 32bit
Intel Core 2 Duo E8400 Wolfdale 3.0GHz
mushkin 4GB 240-Pin DDR2 SDRAM DDR2 1066
MSI P43 Neo3-F LGA 775
MSI NX8800GTS 512M OC GeForce 8800GTS (G92) 512MB
hec ACE-580UB 580W ATX12V.

The games that crash have ONLY been on STEAM (i think). Games like Crysis have never crashed even at the "high" settings mark.
Could it be that I need to take a ram stick out? I have two with a total of 4gb, but XP only sees or uses 3gb of that. Could it be my powersupply?
Could it be i simply screwed up xp and need to reformat (im getting that vibe from some of the things ive done to it).

All help is appreciated.

 

[kagekitsune]

try these steps first.

Update drivers dont forget to clean out your old ones before installing new ones.

Reseat memory make sure everything is in correctly a small incorrectly seated memory stick can cause major problems.

reseat your video card make sure its all the way in on both ends.

record your cpu temps use core temp and turn on recording look at the temps after a lockup if their too high you need to check your heatsink/thermal paste

if it fails try using the 1 stick of memory test it may be a faulty ram stick you can also use memtest to test memory.