No display at POST after upgrading to eah5770


I have reciently built a computer consisting of the following:
Asus p5n-d nForce 750i SLI Socket 775 8 channel audio ATX Motherboard
Intel Core 2 Quad Q8300 2.5GHz Socket 775 1333FSB 4MB L2 Cache processor
750w Cit ATX 12v Psu
Nvidia Gforce 9400 GT
4gig of DDR2 ram
DELL Monitor (VGA)
Windows XP (home) SP3

This setup worked fine for a few days then just stopped displaying anything to my monitor starting at the POST.
I removed my 9400 card and put an old Nvidia Gforce 6800 in and it works fine.
I decided to buy a new graphics card: Asus Ati EAH5770 cu core 1GB GDDR5 card and it, like the 9400 has no display on the POST.
I have put my 6800 back in and it works fine still.

A few things worth noting:
I have put the 9400 into an older cheaper pcworld factory built computer and it works fine and if possible I would rather not put my Asus Ati 5770 into that computer.
I have updated (flashed) my BIOS to its most current update via the Asus website which went well and when checking my bios it lists the new version correctly.
I have updated the chipset drivers from the Asus site to the most recent version.
I have done a clean install of windows (but had to do so with my 6800 in as its the only one thats displaying).
I have attempted to install the ati drivers before putting in the card but it failed during setup due to there being an Nvidia card present
My chipset drivers are by Nvidia and when installing them i allways get Nvidia display drivers too (dont know if this is the problem)
I get no beeps during post with any of the cards.
With the Ati 5770 card it will boot all the way to windows (I can hear the windows login tune, and log out when shutting down) but just cant see anything only a black screen.

Please for the sake of my computer can someone shed some light on this matter it would be greatfully appreciated.