Apple has issued an 8800 GT Graphics Upgrade Kit for Rev. A Mac Pro owners. This upgrade kit has been available for a while now for Rev. B Mac Pro owners, but do to a firmware issue it is only now available to those with a Rev. A Mac Pro.
There are now 2 versions of this kid, one for the pre-2008 Mac Pros, and the other for early 2008 models, so be sure and pay attention when you order, or you could end up with an incompatible kit. The kit runs $279 and is available now.