New graphics card - no video signal

I have what seems to be, from looking at the forums, a fairly common problem with installing a new graphics card. I have been using the on board graphics on my computer since my previous card failed. This was a Nvidia PCI-Express card and when new worked as soon as it was plugged in and the drivers installed.
I have just tried fitting an Asus EAH5670 PCI-Express 1Gb GDDR5 card but I cannot get a video signal to the monitor on booting up either from the new card or from the motherboard video socket. I have updated the bios without getting it to work. I have tried every combination of settings in the bios with the on-board graphics disabled and drivers removed but with no success. All the Nvidia drivers were removed when I had to do a clean reinstall Windows XP some time ago. As soon as the new card is removed, the computer starts up normally with the onboard graphics.

My computer is as follows:
Motherboard: Intel D915GAV with the latest bios
Processor: Intel Pentium 4 520 2.80GHz
Memory: 2560MB
On-board Graphics: Intel 82915G Express Chipset Family
Power Supply: Eagle DR-B400ATX - 400W, 3.3V - 28A, 5V - 4 0A, 12V - 17A

In the bios setup, there are three sections relating to PCI configuration,

1. PCI Configuration where all four PCI slots are set on 'Auto'

2. Video Configuration with the choice of '(PCIE Graphics (PEG)', 'Ext PCI Graphics' and 'Auto'

3. PCI Express Configuration with the choice of setting 'PCIE x16 Link Retrain' to either 'Enabled', 'Disabled' or 'GFX Card'

Dispite doing a fair amount of checking before purchase I am now concerned that I have bought a graphics card that is not compatible with the motherboard. The Power supply is slightly under the 450W recommended by the manufacturer but would this cause the complete lack of any video signal? Sorry to ramble on but can anyone please throw some light on my problem?
 
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