as far as graphics go, the "old" mac pro will support the 8800GT and other newer cards for years to come, since PCI express 2.0 is a load of bullocks and is fully backwards compatible.
To bad according to apple this isn't true. Everyone who ordered the 8800GT(separate) has got a phone call asking to make sure they have the new mac pro. And they even changed the requirements on the store. Technically it should work but according to apple it doesn't. But no one has tested this yet. Many people are very ticked off about it. This thread goes on for pages about it: https://forums.macrumors.com/threads/410804/
To bad according to apple this isn't true. Everyone who ordered the 8800GT(separate) has got a phone call asking to make sure they have the new mac pro. And they even changed the requirements on the store. Technically it should work but according to apple it doesn't. But no one has tested this yet. Many people are very ticked off about it. This thread goes on for pages about it: https://forums.macrumors.com/threads/410804/