Look at this chart:
http://www.tomshardware.com/reviews/quadro-fx-4800,2258-10.html
The GTX280 just can't compete with a workstation card, but hardware wise they are pretty much the same. It's all in the drivers. If you can softmod your card to a quadro and install appropriate drivers, then you'll unlock some (maybe not all) of the performance of a quadro. But then again, this might break support with some games (quadro drivers aren't optimized for those).
It's a bit of hits and misses, and this is what you pay for when you buy those high end workstation cards: High end performance and compatibility in professionnal softwares without all the hacking and testing.
The question is, do you really need a better card? It will gives you better feedback when interacting with the content (moving, rotating objects or views) and gives you real-time representation of useful stuff like bump maps, etc. However it will not speed up your final render.
I highly doubt SLI would bring anything worthy to the table for CAD and 3D software (maybe I'm wrong here though). I'm not even sure quadro drivers support it. I would check before considering it.