I am considering building a pc, and have decided I want an NVIDIA graphics card (either a gtx 780 ti, or a quadro), but I am unsure if getting 2 cards and running them with SLI is worth it. I don't play videogames, but I do a lot of graphic design/3d rendering. Price doesn't matter to me, I just want to know if there will be a noticeable performance increase.
Well if price really doesn't matter to you then I'd advise you to use the money to get a SINGLE high-performance card, rather than dual SLI. Not all applications can be easily configured to run on both graphic cards. The performance increase won't be noticeable if the work you are doing doesn't even need that much alllocation!
More reasons: (Quote from another superuser question)
SLI - require some tweaking, set up to work
Two cards require more power.
Two cards add heat.
Two cards run two fans = a bit more noise.
Most would suggest that one single more powerful card will give you less headache, work with more software, and use less power.
Here's an example(although the subject is gaming):
I just swapped out 2x gtx280 for 1x gtx590 and can safely say it's performance is outstanding while at the same time it is soooo much quieter to run under load versus the old 280sli setup as well as giving the benefits of less heat, lower power consumption and less importantly it frees up a mass of power cabling versus the sli setup as well as freeing up a PCI-E lane for future use.
If you were building a gaming rig, then you should probably go for the dual setup. But if it's graphic design, then it's better to stick with one better card.
EDIT: Yes, as Ramhound said, modern games are designed for SLI in mind, so if you aren't a gamer, go for one high-performance one instead.