It's hard to compare the 512GTS with the GTX, because the GTX is beat in almost every benchmark I've seen except once AA is turned on with incredibly higher resolutions, but the framerates are so low that it's irrelevant. I say the 512GTS goes on top of the GTX, IMO.
Dude, whatever you paid for at the time for that thing (if you bought it new) you could of easily paid for a superior card geared towards gaming.
The Quadro and FireGL cards are meant for workstation computers for rendering 3D graphics and video editing. Yes, they can do gaming too, but they are not meant to.