In your price range and situation, I would definitely go for a single more powerful GPU.
I would also wait a few days to see the GTX 660 (non TI) reviews and price on release. You might like the GTX 660 or 660 TI for that setup. I would recommend a Kepler (600) card since your system looks good enough for it.
Your original setup was kind of weak - low power cards should really never be put in SLI. Part of the power is wasted in the sharing process. Only get SLI if you can't get a single card in your performance range/budget.