The memory size is not a big plus for the fx 5800. You can buy two evga gtx 285 2gb each and get the same amount of memory.
You are mistaken: running two cards in SLI does not increase available memory. Each card retains a copy of all data, so effectively SLI doubles your memory requirements (or halves available memory, depending on your point of view).
The Quadro drivers contain extra options and optimizations tailored to workstation applications. This includes better acceleration for some OpenGL code paths, support for quad buffer stereo and other things - read the product sheets for more information. The gist is that Quadros are better suited “for rendering large medical volumes” and their price reflects that.
If in doubt, you should probably contact Nvidia sales support.