I used to run a pair of HD4950 (1GB) ATI cards in crossfire (similar to SLI). I noticed very little change whether Crossfire was enabled or not. I replaced them with a single HD7970 (2GB) and noticed a great improvement.
Granted, it's somewhat of a case of apples to oranges, as these were older HD4950 cards, Crossfire is somewhat different than SLI, and the rest of my components are different than yours, but I do think you'll see a lot more bang for the buck in investing the dollars you'd spend on a second card elsewhere in your system.
On a side note, if you want to future-proof your system, you may as well get a mobo with SLI support.
Edited by Khlaw, 31 December 2012 - 06:11 PM.