two 6950's in Crossfire would kill two GTX 260's in SLI even if they were max factory overclocked with 216 core.
It would be a good upgrade.
But if you have the right mobo to fit in three cards for triple sli, just find a GTX 260 for about $80 and you should be good that direction too.
You can always sell the three cards as a bundle on ebay/craigslist if upgrading in the future.
But since you are playing 1920x1080, i don't see why you should really jump to 6xxx series.
They have more tesllas for resolutions beyond 1920x1080.
i also use 1920x1080 so i went from 4870 to a 5870. no point in jumping to 6950 and 6970 if you don't use eyefinity and have higher resolutions.
Good luck choosing!