Well, consider also that whatever you go for, there will be a new generation of cards available before the end of this year. This generation is being produced at 28nm, and will likely be the biggest jump in performance since the 4870 -> 5870.
IF i were in the market for high end cards, id take that into consideration.
So if money is any concern, i wouldnt upgrade yet. If its not, id go with a 6970 or a 560ti. Both of those have great performance solo and can be Crossfired later to provide a hefty boost in performance if you choose to go that route. The second card you add is likely to be way cheaper in say 6 months when the new generation of cards is out.
Its not easy.. Perhaps a compromise is to buy a used card of the 460 1gb OC caliber, or the 6850/6870 caliber. Those should be found at around a 100$ and under, used.