Based on your specs you'll have no issues installing a second card.
As mentioned above, there are some negative aspects about running SLI cards.
1. Additional Heat which can be an issue with non-reference cards. Top card may get hot.
2. Incompatibility or poor scaling with some titles.
To elaborate on point #2, most titles will take advantage of an SLI setup and net you a significant performance boost. However, some titles will not be properly optimized and you will not get the performance boost you're expecting.
Most AAA Titles such as BF3, Crysis3, and Metro LL are optimized to properly utilize an SLI setup.
If I were running a 760 right now I would certainly jump on one of the excellent holiday deals and go SLI.