More Computer Lock Ups

Here we go,

This bug has been plaguing me for the past 3.5 years.
As soon as I brought my comp home from the store and booted it up for the first time I installed Warcraft 3. It ran fine for a few hours, but then the image locked up, and there was a sound loop (mixed with a screechy kind of sound). At this point the computer would not respond to anything, and I had to hit the reset button.

I figured it was just a bug with warcraft, so i put up with it. After all, It wasn't that frequent, and I grew up with Windows 98 and ME (unreliable at best). Was I ever wrong.

Since then, I have encountered this issue in the following games:
Diablo 2 (once or twice)
Sims (once every hour or so, less frequent at lower graphic settings)
Sims 2 (same)
Sim City 4 (same)
Guild Wars (very frequently (mostly during the load screens), rendered the game unplayable)
Civilization 4 (same)
Knights of the Old Republic (constantly, every 5 minutes, during play and load screens, coupled with this games own inherent bugs gets messy)
Knights of the Old Republic 2 (same)
Sid Miere's Pirates (same)
Fable (same)
Gun (encountered once, I then discarded the game)
X-Men Legends 2 (occasionally)
There are probably a bunch more that don't spring to mind at the moment

Oddly, The following games play perfectly, despite the fact that many are far more taxing on the old comp than the previous list

GTA, GTA:vc, GTA:sa
Doom 3
Far Cry
Black and White (1 & 2)
Civilization 3
Rome - total war
Neverwinter Nights
Morrow Wind
you get the point

Does anyone know what these games have in common?
Could it have something to do with their reliance on OpenGL or DirectX?
Or possibly the means in which they access ram? faulty ram is one of my prime suspects (see bottom)

I've been through 3 different video cards since I began noticing this problem: Radeon 9600, Radeon 9800 pro 128mb, and Radeon 9600 XT 126mb (currently installed) all 3 encountered the same problem

I've been through 2 sound cards: audigy 1, audigy 2 (currently installed)

I'm using a pentium 2.53GHz, thats currently running at 1.92GHz, 1024MB, with Win XP

I've checked the heat and power Gauges in the BIOS, temperature and voltages are normal.

I've literally tried every single video driver available (ati and omega)
Same with sound drivers
I've patched all the games
I've tried every patch/driver combo imaginable
I've disabled my antivirus during installation and gameplay
I've exited all programs before launching the game

At the moment I have 2 theories.

The first is that this is being cause by a faulty pair of ram chips.
In a thread I found here: http://www.techimo.com/forum/t158653.html
people were discussing similar sounding issues.
Although I ran a diagnostic program called memtest86 which concluded that my memory is fine. I'm still suspicious.

The second theory is that my motherboard may be disfunctional. I'm not sure how to check what type of motherboard it is, but I think its an Asus of some kind. Does any one know of a Diagnostic program to test the motherboard?

Has anyone heard of anything like this?
Does anyone know of a diagnostic program, or crash log or something that might help shine some light?

I'm pretty well ready to accept defeat, but even if it is uncurable I'd like to avoid getting stuck with a lemon again next time I buy a computer.

I wouldn't wish this problem on my worst enemy

 

My mother board is an Asus P4B533