I installed a second x1900xt into my quad-core macpro (to hook up a 3rd, and eventually a 4th monitor), and it worked great under MacOS (two Dell 30", and one 23" Cinema). But in BC Vista64 (which handled the X1900/dual30's wonderfully), it could not see EITHER radeon card, and presented me with a PCIe bus/resources problem. I logged an email to radeon/amd (since Device Manager indicated it was a driver problem), after trying to load catalyst 8.12 drivers.
I had to install the second x1900xt to get 'back' to my vista64 dual-mon setup.
*Anybody got twin x1900xt's to work in BC Vista64?* (if not, does anybody know of a second graphics card that WILL work with the original x1900xt under BC Vista64? I have 15 days to return the x1900 and could exchange for something else that might work with the x1900). Thanks.