Display has distorted colours and resolution issues.

I've done all I can to fix this issue, and I need some help diagnosing the problem.. Me thinks it might be a hardware issue.

New build, here are important specs

750W Corsair PSU
4GB mushkin DDR2
Radeon HD4850
P5Q-Pro
E8400

On Vista 64 bit (and XP 64 AND 32 bit) the card displayed heavily distorted colours and would not let me change resolution after installing ATi drivers. Before drivers I could get to 1400x1050 with no issues. This is with all versions of windows. After installing drivers, however, it would only stick me in 800x600 and nothing else would work.

I stuck in an old card that I knew worked, an old X850XT and it showed same issues. For the heck of it I threw on the VGA cable.. and guess what?!?! It worked!! Everything was crystal clear and I could get to 1680 x 1050 (max reso of my monitor)

So I went to try it with my HD4850, and voila, it works too! So I try the HD4850 in my old PC, and yes, it works perfectly there, and no issues on both DVI and VGA. The cable that produces distortion and errors is working fine, as it was also used for the tests in the secondary PC.

I think it's the mobo, but I can't put it in another system to test, as I only have one LGA775 socket CPU and only one set of DDR2 RAM.

Any help would be greatly appreciated!