If you have 10.5.2 or later installed, it's a rather simple matter to install a 8800GT in a Mac Pro. Note, do not get rid of your old video card. If you ever need to clean install Leopard, you will need the old card. The Leopard DVD does not contain the drivers for the 8800GT - I learned this lesson the hard way last weekend.
Installing the card is a matter of opening the case, loosening 2 screws and removing a bracket that holds the old video card in place, and unplugging the old card from it's slot. Note, there is a little flexible plastic lever on the slot that sort of locks the card into the slot... just gently pull the lever to the side and the card should come out with minimal force.
Once the old card is out, it's time to put the new card in. The only tricky part about this is getting the power cable plugged into the motherboard. If you have the old 1900 XT, that cable already exists and is plugged in. Make certain you leave it plugged in when you remove the old card, it makes it easier. Since you have a first generation mac pro, you probably have a 7300 GT, though, which does not need the power cable. The power cable that I speak of comes in the box with the 8800 - along with installation directions, by the way. You need to plug the smaller end into the mother board and the larger end into the 8800 GT. Then just plug the card into the slot you removed the old card from. Note, make certain to plug it into the bottom-most slot. Even though they all look the same, only the bottom-most slot runs at full 16x speed.
Once the card is plugged in the slot, and the power cable is connected between it and the motherboard, you just need to re-screw the bracket down, plug in your monitors, and you are good to go. The mac should come right up with no problem.
Windows, under Boot Camp, is another matter. I suggest going to Add Remove Programs in XP, or Programs and Features in Vista, and uninstalling the ATI and nVidia drivers that boot camp installs.
Reboot back into Windows and your card should come up in VGA mode. To be thoroughly safe, you should download "Driver Cleaner Pro" and use it to remove all the left over drivers and registry settings from the nVidia drivers. It seems that going forward from the 16x series of drivers to the 17x series will sometimes leave behind part of the 16x series. "Driver Cleaner Pro" is an easy to use and free utility which will take care of this issue. But only run AFTER you have uninstalled the old drivers in the normal manner.
Once you are certain the old drivers are uninstalled, you can get the latest from nVidia (currently 175.19). Just download these drivers and they should install without a problem. Once you reboot, you are good to go. If you are using Vista, you may need to re-enable Aero, if you care for that theme.
I hope this helps