Ati Radeon Hd 3870 X2

At work the Nvidia graphics card was failing on a first generation Mac Pro which didn't have AppleCare. After doing some research on compatible graphics cards we purchased a PC/Mac Radeon HD3870. The software would not install from the CD saying that a newer operating system was needed (because QuarkXpress 6.5 is still being used, Tiger is installed). Anyway, I installed the card anyway and it worked. However, when one tries to play a DVD one gets a message that 'A valid video device could not be found for.' This isn't a great inconvenience but does anyone know if compatible drivers for Tiger are available for this card; I've done a google search and looked on the ati site and have drawn a blank. I have also suggested that the graphics designer upgrade QuarkXpress to the latest version which only costs a couple of hundred pounds and would allow an update to Leopard/SL but he doesn't seem interested.