Overheating / BSOD / Page faults = MoBo or RAM?

Hello,

I've been having this problem for months now and I just bought a $250 video card because I thought my 3-yo GeForce 8500GT was the cause.

Here's what I get whenever I try to do anything CPU-intensive, like playing TF2/other games:

- Motherboard overheating by 5-10 degrees (114F on idle)
- Data Execution errors whenever I quit a game (says the process has been terminated to prevent system damage)
- BSOD "Page Fault Found In A Non Paged Area" Error 50 (I have the minidump but not sure how to read it)
- I usually leave my PC on Sleep while I'm away but if I ever do turn it off and on again it takes me 5-10 minutes to get it to POST. It doesn't give me a beep and the monitor doesn't get a signal. I usually have to unplug the HD (mostly to avoid errors on it from all the force-restarting) then it works.
- 10/13/2011 2:26:35 PM, Error: Application Popup [1801] - The hardware has reported an uncorrectable memory error.

Here are my system specs (Custom built, 3 years old):
Intel Core 2 Quad Q6600 @ 2.4Ghz
8GB DDR2 Supertalent RAM
64-bit Vista Sp2
GeForce 8500GT
MoBo is an ASUS P5B SE
(NEW) Corsair TX650 PSU
2 fans + 2 extractors + 1 hd fan


- 3 months ago it had a faulty DDR2 module and I got a replacement from the manufacturer. I didnt try running a memtest after that because it came in a new pack. The other 3 modules gave me no errors.

I've googled about the BSOD and it mentions mainly RAM, VideoCard, MoBO as culprits. I need help narrowing it down because I can't replace the whole thing right now.

I'd really appreciate any help on this matter.