Re: So I added a second GTX 680 4GB in SLI. Here's how it works with modern games.
There is a third slot, but with these big cards two is all I can fit. But even if it fits, it wouldn't be good. The CPU can't handle a third one (I'm already getting a bottleneck in one area in Dark Souls 3), the mobo's chip set is a Z77 with limitations that three cards will have the third work at PCI-E X4 only which will hurt the performance of a GTX 680 so it won't be cost effective, and actually the gain will be little if not worse due to the above and lack of support as per the reports. A second card add ~80% performance at least in all games I tried. A third adds no more than 20% at best, if it does, that is.
Z77 chip set can run a single card at X16 speed, two card at X8/X8 and three cards at X8/X4/X4. The third option will hold the second and third cards back if they are of high bandwidth and require more than X4 to run at full potential.
Low profile and bandwidth cards could work tho and would be a good choice then. But that's not economical or worth it now. Maybe 5 years ago it could have been.