Hmm... considering your memory passed memtest after such a long time, I would be tempted to think it's fine.
One of the first things I would usually check are the CPU temperatures and system voltages (though motherboard voltage ratings are to be taken with a pinch of salt, especially since some boards report them incorrectly)
There's a program called motherboard monitor, it's a lifesaver for me.
http://www.pcworld.com/downloads/file_d ... &fileidx=1
basically it can be set to sit in the system tray and display the temperatures.
Now I've never owned an Intel, so I can't comment on what are safe temperatures, I suppose you will have to revert to google or try here
http://www.hardforum.com
I notice you are running SP2. Are the problems recent, or have they been occuring since you've had the computer?
Also about your stop errors, do they appear at boot, or during use?
Just looking at your specs, here are a few things that I would personally check out:
Service Pack 2 - Did the errors occur before SP2, or have they always been there?
Maxtor hard drive - How old is the drive itself, and does it get daily use or had a fair amount of wear?
Radeon 9800XT, check the temperatures. I believe this card automatically overclocks itself but monitors the temperature, are any cables on the card damaged? Is temperature/overdrive switched on in the driver? Try uninstalling the driver and go back to a safe version like 4.2 and see if that helps
Integrated devices - If you have onboard video and the 9800XT, make sure the integrated video is disabled in the BIOS.
As I mentioned, I seem to be experiencing problems with the cats 4.9 and SP2, but since its just a 9800, its not overclocking itself and I dont plan to just yet (this is just a tempoart install until I get a new mobo)
Well good luck, I hope that works