Basically the way AMD/ATI video card schemes currently work is the first 2 digits indicate the series of the card. 3x, 4x, 5x, 6x (in mid January 7x) the last 2 digits indicate the performance level. In both cases higher is better (generally speaking).
In this case the 6x number tell us it is 3 generations newer than a 3x and the 70 indicates it's higher performance card than the 50.
If you want benchmarks, look in the charts sections. If you want specs, check the retailer.