ATI Firepro v4800 3 monitor support

Are all three monitors exactly the same on both computers? If not, you may be hitting a resolution limit.

The V4800 is an Evergreen-era GPU (actually one iteration later) so I'm a little surprised that it doesn't support up to three 1920x1080 displays...

I admit that I am not very familiar with the differences between dual link and single link DVI, and that is something I have not verified is standardized between the two computers. Is that a possible reason one is working and the other isn't? How do I check if the DVI cable is a single link, or dual link?

Single link DVI has "missing square pins" or "missing holes" (depending on whether you're looking at male or female connector).

Dual link DVI has square pins all the way across.

The difference:

Single Link has a lower maximum resolution (or lower refresh rate if higher resolution) because it can transmit less data with less pins. Dual Link can transmit the highest maximum resolution / refresh rate. Cable lengths also can affect resolution.

For a full description, see Wikipedia.

Uninstalling, and completely removing all of the driver files with the Driver Fusion utility recommended here, and then reinstalling driver 8.773.1.1000 seemed to do the trick.

I suspect that installing the newest driver would have worked as well, but I knew that the older driver worked on the other computer, and I didn't want to risk having to do the process again.

Thanks to Hennes for recommending this approach, and allquixotic for the info on DVI,