Whiteknuckle,
See my previous post...the 8800 is the choice right now if you will be gaming. Keep in mind, you need a copy of Windows as well for Bootcamp. And if you decide to go with XP, you won't be able to take advantage of Direct X 10 (not worth it right now, IMO regardless).
If you are using your MacPro as a Windows only machine, you can use any video card you want. If you are wanting to use the same video card on both OSX and Windows, your options become limited...but the 8800 offered by Mac is in my opinion, coupled with this machine, faster than any pair of 8800 Ultras on ANY gaming PC rig available at ANY price!
A couple of friends of mine just built the new BlackBird HP rig with twin 8800 Ultras on the Extreme Quad Intel 9850 platform and it runs like a DOG!!! At less than 15fps and at a cost of around 6k!
The offered option from Apple is well worth the $200...but as you will find out running FSX on a Mac Pro...>FSX runs better and more efficiently on Raw CPU horsepower, NOT GPU horsepower 🙂
Take care,
Jeremy