A single 6950 is about equivalent to two 4850s. You'd get roughly the same performance, but you would get DirectX11 support and the single 6950 would probably use less power and generate a lot less heat than your Crossfire setup.
The main difference between the linked cards are the coolers. The first one has the reference cooler which is on the noisier side and not as good at dissipating heat. The second card you linked has a custom cooler designed by HIS that should perform better than the reference cooler, and hopefully be quieter.
If you do want to get a 6950, go for the cheaper 1GB version unless you intend to game across multiple monitors of play at a resolution higher than 1920x1200. The 2GB version only sees a performance improvement at extremely high resolutions. The 2GB version also has the chance to flash into the 6970, but most 6950s manufactured now have the extra shaders laser cut to prevent people from unlocking their cards into a 6970, the chances of a successful unlock are getting quite low.