I purchased a GeForce GTX with 768MB of GDDR3 for our 2008 Mac Pro. It not only has the same chipset as the Quadro FX 5600 but purposely chose one whose core and memory was overclocked to match the frequencies as the Quadro FX 5600.
Of course, it won't run under Mac OS X, but it screams under Vista Ultimate 64-bit. I ran our Prey benchmark at 1920x1200 "High Quality" and got 166 fps.
I plan to benchmark the GeForce 8800 GT under Vista once our kit arrives to see how it compares. Meanwhile, a friend tested Prey on his 2008 Mac Pro with the GeForce 8800 GT and managed "only" 91 fps under Mac OS X 10.5.2 with the Leopard Graphics Update installed.
I'm told that if I flashed the GeForce 8800 GTX with the ROM from the Mac version of the Quadro FX 5600, it would work under Mac OS X. Think of it: A "Quadro FX 5600 kinda-sorta" for $550.