Graphics - Major Problem

Hi people, thanks for looking

My specs

ABIT IX38 QUAD GT
Q6600
LEADTEK 9800 GX2
CORSAIR 750W PSU
OCZ 4GB REAPER EDITION 2X2GB 1066MHZ

Right, since I upgraded my motherboard and ram I am having a major issue. Everytime I play a graphically intense application such as RA3, HELLGATE LONDON, COD4, 3DMARK 06, it will sometimes get around 15 mins into the game and then the screen will start to flash black. Then after a while it will say "display driver has stopped working and has recovered" or I will get a BSOD pointing towards the nvidia driver.

Sometimes I can run 3dmark06 and some games fine but usually it will get the graphical problems, I have even had graphical corruption on some and like a white fuzzyness on the screen which only disapears when I reboot pc.

I have installed all the latest drivers for everything, so everything is up to date, even tried the new beta drivers for my 9800 gx2 but its still the same.

All temps in my system are good so thats not an issue.

Reintsalled fresh copies of Vista and XP and its still the same.
Ran memtest for 8 hours and no errors.

I RMA' d my graphics card buy they said it wasnt faulty so they returned it to me.

I really need help here, never encountered anything like it before