The 4850x2 is more powerful, but that doesn't necessarily make it better. Single cards tend to be more consistent in performance, use less power, and a better single card has the option of Crossfire later.
That being said, it comes down to price and what resolution you play. At resolutions of 1680x1050 or below, a single 4870 will do great and the price difference wouldn't really be worth it. Higher than that, I might consider the 4850x2 instead.
Are you saying two 4850's, or a 4850X2? either way, the 4850s will outperform a single 4870. But, this depends on the system its going in, what motherboard, cpu.. ect will it be paired with?
Are you saying two 4850's, or a 4850X2? either way, the 4850s will outperform a single 4870. But, this depends on the system its going in, what motherboard, cpu.. ect will it be paired with?