Having gone from an Asus 7770 to a Diamond reference 6870, I can say that at idle there isn't much difference. Both were barely audible. It was only when the fan goes above 50% that I really noticed the difference. The fan seemed efficient enough in the stress tests I put it through, but it was significantly louder under full load. Exactly how much is hard to say, but I was a bit surprised at just how loud it got (by comparison).
Personally, I don't stress my cards out often enough to care about the difference in noise, especially when I have other sounds (from whatever game) that easily cover it up. But good luck to you and your decision!