You are right in that those Quadro cards are not for gaming and as for whether it can run GTA4, it may or may not. if it dose then its performance/FPS will cirtainly not be up to par with an equivilant gaming version of the card. if you already have the card you could try it, but dont expect much.
the best/popular place to sell it would be ebay, maybe only expect to sell at around 1/4th of buying new.
what screen resolution do you play at? what are the components of your computer? how many watts is your power supply?
My computer is barely new. Has a AMD Athlon II X3 2.7GHZ processor. 2GB of memory but going to add 4GB in one of this 2 weeks. A Biostar Motherboard. The Quadro FX 3400/4400 Graphics Card. 450W power supply. Screen resolution is 1440x900.
Running Windows 7 64 Bit.