Comparing clock speeds for different types of cards is not meaningful. You need to look at benchmarks.http://www.anandtech.com/bench/Product/540?vs=510
The 6950 is simply faster than the 6870. Also, more VRAM is only beneficial in some games and at high resolutions with AA/AF cranked up. For most games, 1GB is plenty. In fact, 1GB 6950s tend to outperform the 2GB 6950s by a small margin, around 1%, in many games.