The 6xxx series offers a faster high end and more choices on the mid ranged. The most notable increase in performance was for tesselation support in dx11. So if you are wanting to play games such as metro 2033, crysis 2 and such in dx11 with tesselation enabled, the 6xxx series would be the way to go it all depends on your gaming needs
Sorry on the brief reply was going out. As a comparison the 6950 is comparable to the 5870. The newer card gives more up to date DX support as mentioned. Much of the 5k series has been phased out. Due to the changes in design and manufacturing you get better performance from slower clock rates and lower power usage (=heat) in the newer cards. All that said you can make some great buys on the older cards if you look around. In a few months, AMD will have a new line of cards and then the 5k series will be further down on the charts.