Until now, if you had an older Mac Pro desktop computer from Apple, belonging to the first generation, the video card upgrades available were rather limited. The fastest mainstream graphics card available for these first-generation Mac Pro owners is still the older ATI Radeon X1900 XT, but that will change pretty soon...

GeForce 8800GT cards for Mac Pros are already available in Apple's online store, but their firmware requires the computer to have a free PCI Express 2.0 slot, and older Mac Pro computers don't have one. The strangest part of the problem is that most PCI Ex 2.0 cards on the market at this time have no problems working on old PCI Ex slots, but the 8800GT available to Mac users is an unfortunate exception...
The new Mac Pros come with the ATI Radeon HD 2600 XT card by default, and this one is far less powerful than the 8800GT, and even than the older X1900XT mentioned earlier, so it's expected that NVIDIA's decision to work on making the 8800GT compatible with older Mac Pro computers is a very good one.
Unfortunately, there is no clear information on when this is going to happen, but according to sources from NVIDIA, the new 8800GT for Macs should be available for purchase on Apple's website "in a few weeks".

Matt Ringling, on March 20, 2008
It's nice to hear a definitive answer on this as I've been looking for weeks. I have the first generation Mac Pro, and I am going to upgrade to a 30' HD monitor in two months. But of course the standard card doesn't run such things.
So I have been looking for cards and thought I was in luck with the 8800GT, for performance and price.
But then I found out about the PCI 2.0 issue. But the confusion over that being backwards compatible with standard PCI was making it difficult to make a decision.
So I am quite thankful for a straight answer on the matter. I hope you will be updating this when the actual product comes out, because I'm sure Apple will not make a large announcement as it's kind of fixing a mistake they made.
Much appreciated.
Matt