No one's going to know this better than you. Test your in game performance in multiple titles - but using just one of the gpus. Monitor the usage of:
-each cpu core/thread, because one can be cpu limited on just a single thread
After the multiple tests are done, does the cpu prove to have plenty of headroom to run a 2nd card, or is it constantly high, thus implying that the addition of a 2nd card would make things worse?
-Do the games you play officially support SLI, or is it some half-assed patch?
-Is that a HB(high bandwidth) Bridge, or just a normal one?
-Do you have a good quality 650w or greater psu to use with such a setup?
-Is your main monitor's resolution greater than 1080p?
If the majority of the answers to the 5 questions are no, then I'd suggest not bothering with SLI, as it'll likely do more harm than good.