Dude, I think that your understanding is wrong:
1) The 3GB card is 3GB for NVIDIA 3D surround and is no different than the 1.5GB version if you are gaming on a single display.
2) The Quadro CAN NOT PLAY GAMES, people think that just because the Quadros are used to make the games, they can play them, WRONG!
The GTX 580, 570 and 560 Ti ALL have faster GPUs than the Quadro, Quadros will perform bad in games, the reason they are expensive is only for the drivers and will LAG in games. Quadros are only for workstations!- they are not at all over kill and can be beat by any card for less than $150 in games,
A Quadro 6000 will give you the GAMING performance of a GTX 460 for 30x the price!