None taken. Your assumption that the Quadro is better for games is only because of one set of benchmarks out there. There are plenty of tests on PC that show the quadro trailing their sister gaming cards.
The reason the Quadro is faster in OSX for games at the moment is due to the fact that apple has had time to tinker with those drivers longer. Give it 6 months or so and a few updates and you'll see the 8800 ahead.
Side note, some windows testing here shows actually that the 8800GT is quite comperable to the FX5600 for games. Wins some and loses some and seems to be about tie on others.
http://www.barefeats.com/harper11.html
Anyway it would be silly to get a Quadro for an older Mac Pro. A the price that Apple is asking, it may as well be better to sell that box and buy a new computer with the 8800GT.