A user in the French MacBidouille forums claims that he was able to install a Saphire ATI Radeon 2600 card into his new Mac Pro without issues.
The card had not been tested alone, but this is very strong evidence that an unmentioned update to the Mac Pro is the ability to use any graphics card. When NVidia 8800GT card in the new Mac Pros was announced as incompatible with the earlier Mac Pros, Mac Scoop points out this could be because the card is in fact a standard card that could be used in a PC just as easily.
Before the new Mac Pros, the only way to upgrade your GPU was to buy a rare and expensive Mac-compatible card or to buy a regular card and flash it’s firmware using a PC, which could render it useless.
Read [MacScoop]
Driver?