XFX 7800 GTX not working?

Hi, newbie poster here but I bought my new graphics card based on the rankings on this site.

I bought it from eBay. I used to have a Radeon HD 2400

My bits:

• ASRock 4core1333-FullHD - don't know if that's even the model name, the box has so much crazy fontage all over it it's hard to tell :-\
• INTEL CORE 2 DUO E6750
• 4 GB DDR 2 667
• 400 GB 16MB CACHE SERIAL ATA HARD DRIVE
• XFX nVidia GeForce 7800 GTX 512 PCI

I've tried it with and without connecting both of the power leads to the two remaining sockets in my comp. Ive also tried connecting to both of the DVI outs.

When I switch on, the fan on the card comes on but nothing's sent to the monitor. I've tried re-inderting my old card just to check I didn't break something and it works fine.

I've also tried going into Vista safe mode with my onboard card to try installing the drivers (I have the ATI drivers still installed) but it says it won't install as it can't find any nVidia hardware. Whether this is because it actually can't find it or just because it's usingt the onboard and doesn't register the card which is not currently being used I don't know.

Anyone ever experienced something similar? Or is my card just broken?

Thanks in advance