Consistent crashing in most all of my games [new GPU]

Current System Specs:
AMD X2 3800+
GeForce 8800GT OC
2gb DDR 3200 RAM
1tb HD, 250gb HD
Sound Blaster Audigy 2 Value
Windows XP Pro Service Pack 2
500watt power supply (maybe 450, crap, I forgot).

Okay, so I recently upgraded to a GeForce 8800GT OC. However, ever since the upgrade, my games have been crashing after about 40 minutes of play. So far I've experienced this in both Oblivion and Crysis after about 30-40 minutes of play, and Team Fortress 2 within the first few minutes. STALKER has yet to crash.

Here's the crash though: I'm playing until all of a sudden, the game freezes, then stutters usually going about 1 frame every minute or so, with the sound acting really wacky as well, sometimes going out, sometimes making weird noises. The only way to fix is to manually restart.

My old graphics card, a GeForce 6800GT OC, used to have an overheating problem where it would start showing wacky effects then completely locking up. I thought this might be the same issue, but I get no wacky effects beforehand, and this never COMPLETELY locks, it just mostly completely locks.

I also thought it might be a PSU problem, not big enough or something, but how could the game let me play it for 30-40 minutes without a problem? Doesn't make sense to me.

After installing the 8800GT OC, I started noticing the problem right away in Oblivion. However, I reformatted (not because of this problem), re-installed everything, and it still persists. I'm thinking it could be a sound card problem, but I never had this problem before in either Oblivion or Crysis, so I'm not sure.

Could it have to do with running a DX10 card on XP? But I don't have DX10 installed (obviously, it's not possible), so I wouldn't expect that to be it.

Thoughts? Help?

That's a lot of stuff to be running on a 450 watt psu.

Can you put your old Vid Card back in and see if you still get the problem?? If you don't get the problem with old Vid Card, then we can rule out allot of the other hardware... Then the problem would lie with the PSU and/or the 8800GT OC and/or drivers and/or heat.... Let us know...

That's a lot of stuff to be running on a 450 watt psu.

Yeah, but could that really be the problem? Considering it runs for a good 30-40 minutes without a problem, I don't see how it could be the PSU.

One other note: I played the SP of Call of Duty 4 at full graphics and beat the whole game in two sittings with zero crashes. STALKER lacked crashes as well. It seems to be limited to games that I get lower FPS in, which to me says heating... but at the same time, TF2 crashes and I don't get FPS problems in that.

I'm really not wanting to put my old GPU in, but I can if need be.

Yeah, but could that really be the problem? Considering it runs for a good 30-40 minutes without a problem, I don't see how it could be the PSU.

One other note: I played the SP of Call of Duty 4 at full graphics and beat the whole game in two sittings with zero crashes. STALKER lacked crashes as well. It seems to be limited to games that I get lower FPS in, which to me says heating... but at the same time, TF2 crashes and I don't get FPS problems in that.

I'm really not wanting to put my old GPU in, but I can if need be.

Sounds like a heat issue, certainly.
Run a program that logs diode temps and voltages to a file while playing.
After the crash, open the file and see if there are any anomalies in the values.

What's a good program to use?

Core Temp would be the best bet for the temperature sensors.

Let me google for some good voltage logging.
e: A quick glance shows not much, but it was a vague selection of terms. Does CPU-Z have a logging feature that allows selective logging?

Yeah, but could that really be the problem? Considering it runs for a good 30-40 minutes without a problem, I don't see how it could be the PSU.
It most certainly is most likely the PSU. A PSU puts out its peak power right at startup. As it heats up, its power output drops. After about 30 minutes you're pretty much at the minimum that the thing will put out, and a poor quality PSU can put out 50-100 watts below its rated capacity once it's fully warmed up.

People always skimp on the power supply. Don't. Most stability issues are because of bad power.

tell us what brand power supply you have and we'll tell you if it sucks. ;)