The 2006 Mac Pro (which is sounds like you have) uses EFI32, which really limits your graphics card options. The 2008 models use EFI64, and have much broader compatibility.
In addition to ATI's 1900 XT, the ATI Radeon 3870 is supposedly backwards compatible with your Mac Pro 1,1 model. Apple released a custom NVIDIA 8800 GT for the 2006 Mac Pro, but good luck finding one. If you get any of these cards, make sure that you are buying the Mac edition. A PC graphics card will not work out of the box.
The graphics card really isn't a factor for After Effects.
Walter Soyka
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