You might be able to find a hack that will make the system think that a GTX card is an equivalent Quadro card. That would give you the best of both worlds, provided it works. If you can't find the hack, or it doesn't work as advertized, then you are probably screwed.
I don't know anything about running two graphics cards, but I seem to recall seeing a thread in the not too distant past that mentioned someone having difficulties, but I think they were trying to get SW to use both cards, which is probably not something you need. You might try a search.
Thanks Jerry. That's a good thought to look into a hack. I'm afraid of problems that I may run into though since I depend on the system for my business. My current thinking has moved to using my old computer for rendering by replacing the old Quadro FX card in it with a single new GTX card. Then my newer primary use computer will have the Quadro card. - Chuck