First off: I'm running my system with a SoftQuadroed Geforce 4 Ti4600, and it's coming along nicely, but some things like hardware accelerated lines (incl. AA) are not supported by GFs, even if the drivers are hacked with SoftQuadro.
As to the gaming capabilites: the Quadro offers full DirectX support, and there are a lot of benchmarks around that show that the Quadro beats the Ti4600 even if it comes to high-level gaming.
Another issue with the SoftQuadro hack lies in the fact that you can't use any drivers above v41.03... I'm still using 30.82 (Maya 4.5 certified drivers), for that matter.
If I'd put togehter a new system now, I'd go for a Quadro or the new FireGL (that said, I don't do any gaming).
Anyway, it's up to you...
Subdivide and conquer!
Free your mind, and your ass will follow!