The most likely cause: some hardware issue with the card. I'd try another one.
I have a DELL Dimension E520 with Win XP SP3. The on board graphichs card is disabled and the PCI Express card was installed (Radeon ATI X550) it has been working fine for over a year, I then got an error
saying ATI encountered and Error (VPU had encounted a problem and reset your graphics acceleration as it was no longer responding). I then uninstalled the ATI Driver and reinstalled. It worked for a day and the same problem occured. I did the same but this time I can't get any display at all, checked the BIOS settings thats OK. I then removed the Radeon ATI card and used the on board that works!!.I then re installed the Radeon ATI card back BUT did not connect the monitor to it the display did not come on. I then plugged the monitor in the Radeon ATI Card and the display still didn't come on. The bottom line is now it only works with the on board graphics card , and if I insert the Radeon ATi PCI Card I simply done get any thing displayed on the screen weather the monitor cable is connect to the on board OR the ATI Radeon PCI Xpress. And power is OK ans the cable if OK. Only works if nothing is plugged to the PCI Xpress slot?? Can any one please help me out .