I have 6950 in crossfire, the first card is at x16 but the second slot only supports x4, is it worth getting a new motherboard? What performance would I expect to see if both ran at x16?
In most cases not much. There might be a few games here and there that can use it, but even then it will be at best 10% faster. I vote not worth the money. Take the motherboard money, sell the two 6950s, and buy something really good.