If the cards are running hot, the fans would have to spin faster ergo more noise. It depends on your case airflow and ambient temps
I've checked a few benchmarks and reviews and all of them said the noise levels for 6870 were very low, even on max load. Only a 1-3 dba more noise compared to normal 6870.
Now Im only worried about the microsluttering, its been said that the 6800 series is a great Crossfire combination to go to but I still dont know in what im getting. Can anyone explain who has 2 cards in either SLI or Crossfire