The 6870 is like a GTX560. The 550TI even if OC'd isn't as fast. 6870 is the faster card.
Scott, whats the point of linking that site? What does any of that have to do with anything? How much faster is a card over another if it has an extra ~48MBps in memory bandwidth. Having raw stats like that is usually next to useless. For example, you might think from this that the GTX550TI is much better then the 5770/6770.