Just installed a nVidia 8800 GT video card in my 1st gen Mac Pro 2.6 Ghz. All I have to say is WOW. What an improvement! All scores are up and games run effortlessly at 1600x1200 or 1920x1200 with Win XP. The Windows Vista performance rating went from 4.2 to 5.9. The computer runs quite a bit faster with this card and as an added plus I can modulate the cards cooling fan with Riva tuner. The upgrade to install this card took all of ten minutes. If you use the computer for Leopard applications I didn't see any improvements when using the OS. Althought I admittedly don't use the OS for much more than adjusting the computer's fan speeds and computer maintenance.
The Apple website says that the video card uses 1 auxiliary power cable. I am assuming this is a pigtail off of the main power supply within the Mac Pro. When I look inside my Mac Pro (1st gen., early 2008), I don't see any power cables - Apple has them well routed somewhere within the box, or just doesn't make them available.
Did you run into problems with this?
P.S. - if I go with the Quadro FX 5600 - the website says I need two power cables...