4870 no video

I changed the cooler on my HD4870 to an HR 03 GT. I booted it up it worked as usual, except that the VRMs became very hot with the HR 03 GT heatsinks. So I removed the card, removed the VRM heatsinks, and ordered new heatsinks. I put the 4870 in a box for a few days while waiting for the heatsinks to arrive. I used a 7800gt in its place.

After installing the new heatsinks and setting the system back up with the 4870, I can't get the card to display any image to my monitor. I've tried both VGA and DVI connections to different monitors. They work with the 7800gt, but not the 4870. I've tried putting the card in a different computer, but it doesn't work. I tried replacing the heatsinks and reseating the cooler, but still nothing.
If I pull the PSU to CPU power connector, the 4870 will, as it did before I replaced the heatsink, display 3/4 red diagnostic lights on its back. With the connector in proper place, the 4870's red lights flash 3/4 briefly(less than a second) before fading out. This seems to me to show that the problem is beyond the card's diagnostic LEDs.
I've poured over every mm of the card with a magnifying glass, but I didn't see anything that looked inconsistent, burnt, broken, scratched, etc...
The GPU's heatsink still gets warm when turned on.

I've tried putting the 7800gt in another slot while keeping the 4870 in its slot and booting up the computer. When I check the device manager in Vista, the 4870 isn't detected.

Are these results consistent with physical damage of the card?
Could the video bios have somehow become corrupted?

Thanks for any help