I believe I have a problem like that before. I have radeon x1900gt, It works well to me till i encounter a problem one day I opened my computer. My computer didn't have a display, I look for the problem. When I use the integrated video with my motherboard, I have a display. But when I return with my Graphics card, it wont give a display. Since my motherboard is still under warranty, I brought my computer to where I bought my motherboard coz I suspect that the problem is PCIE bus. I'm right, the PCIE bus is broken.
It could be your AGP or PCIE bus is broken. Better check if there is a need to put an external power supply. What I mean, maybe there is some power connector from your graphics card that has to be connected to the power supply. Coz some graphics card has its own power supply connector. Like mine, has a pcie power connector.
Hope this helps.
as cool as ice...
