Stop errors while gaming

Hello everybody
I gotta say that i'm a bit desperate. 9 months ago i bought a new rig. the specs were the following: 2.4 quad core 2, ASUS 8800GTS 320mb, 2x1 GB RAM, 500W PSU.
Here are the problems:
While playing Call of duty 4 ONLINE(not during offline play) i either get a Stop0x50 or a IRQL_NOT_LESS
While playing World in Conflict both online and offline i get BSOD's
Very rarely while playing Medieval 2 total war ONLINE i get a bsod pointing out that nv4_disp.dll caused the stop message.
I have tested my ram, i have tested my video card, i have tested my hard disk and tried reinstalling many drivers for the video card. unfortunatelly, nothing helps. i finished a game called Crysis which has very very high requirements with no problems and i can chat, search the internet or watch videos for 48 hours with no problems.
sometimes, like once a month my pc would suddenly start roaring while im doing whatever and it would restart. other times, during startup it usually starts quiet, heats for 2 seconds and goes quiet again but sometimes it doesnt go quiet again and my monitor and mouse dont start up with the pc.

i;d appreciate your help