The Quadro cards have great OpenGL support, so they're good for pro 3D and Mac-specific apps/games with OpenGL optimization.
Yes, it'll be great for games as it's an insanely powerful card.
However, it's a waste of money. For the same cash you can get 8800GT cards that will best the Quadro (at least for D3D gaming) for a fraction of the cost.
If gaming's your thing, then build yourself a nice Intel Q6600 system (or wait for AMD to sort out the Phenom issues) with SLI 8800GTX cards. Yes, you're going to have to load Windows, but to play most games you're going to have to do that anyway.