Do you want an SLI recommendation or a single card?
The overall upgrade right now is more important to me than whether it is an SLI setup or a single card. (The mobo also supports Crossfire but I only have experience using SLI.) I would prefer SLI due the increased longevity before needing another upgrade but if the recommended card is quite expensive at this time I am willing to consider adding a second card in the future.
It is my understanding that SLI functions better when the cards are a 'matched' set, if anyone can comment on this point as well with regards to making this upgrade it would be beneficial in my decision.