@free_electron - not always the case.
like i said : same pcb layout can hold multiple chips. some are actually different , some are fusebits.
A key element of Quadro is the drivers. if you got a regular card and you experience a software glitch nobody gives a rat's ass.
If you have a quadro and there is a glitch with a particular software , it will get fixed and it will get fixed very quickly.
quadro's are workstation card. meaning they are used to visualised data from engineering software. solidworks, catia , adobe .. all the big stuff out there.
there are other elements on quadro . OpenGL is run in hardware on a quadro s opposed to software emulation on a non quadro. Quadros can have much more memory than other boards (which is important for massive polygon scenes . And no SLI does not share memory ... ) Try loading a complete Catia design for a wing of an airplane down to the last nut and bolt. change the viewport. The Quadro will respond immediately. the GTX will curl up in a corner ,choke and may give you a screen refresh a few minutes later. unworkable if you are designing.
As software is being rewritten to take advantage of CUDA the quadro will loose its advantage. but tright now , 3d cad software is , in a lot of cases, still driven on OpenGL or DirectX and there the quadro runs circles around the other boards.
in short : for gaming , video processing : no advantage , stick with gtx
for anythin involving massive vertex count : quadro will chomp the gtx to bits. so if you are thinking to make the next Toystory or Monsters Inc , or run things like 3Dmax, Catia and other high end cad software ... quadro is the way to go. games have no advantage on quadro as the amount of polygons a game throws at the GPU is simply too small to notice an effect.