You do have to admit...the concept of a desktop computer that can ONLY use 2 different cards is rather silly.
It would help if Apple would offer more useful info on which cards work in the which machines.
Fact is, the 3870 should work in Nehalem, as should 2600XT, and likely the Nvidia 8800GT EFI64 and the Quadro 5600. Even the Hair Dryer X1900 should work in them. (though possibly with no Windows support)