Problems with Dell Dimension 4600 and new GEForce 6200 512MB graphics card

I recently purchased a new wide screen monitor. As the onboard graphics of my 4600 don't support the 16:9 format I wanted to upgrade with a video card. As far as I can see from the specifications required this card should be compatible - albeit requiring an upgrade to the power supply unit.

I have done the power unit upgrade (using an Arctic Power 500W supply) - works fine. But when I come to install the card I am having major problems:

Specifically, after fitting the card in the AGP slot I can't get any kind of signal from the pc to the monitor. This is the case whether I connect the monitor to the new card, or to the old onboard slot. However, if I remove the card from the computer and plug the vga cable into the onboard slot then I get the normal graphics back. It seems like the system knows the card is there but is not recognising it or it is not sending a signal to the monitor.

I wondered if pre-installing the new card's driver from the website might help but this interrupts part way through the install process  as it does not recognise any NVIDIA hardware, and of course I can't do the install with the card in as I have no picture.

Any help would be gratefully received.

Thanks,

James.