Single 6870's mostly have no trouble... sometimes you need to under clock the memory and then shut down msi afterburner and start it again... I think you might also need to add some lines to the MSIAfterburner.conf file.
My personal experience with 6870s (I'm running a couple of them right now) is that if you have more than one, you will need to flash the bios and use linux.
if you have a look at this topic I'm just about to add my report on doing so.https://bitcointalk.org/index.php?topic=62894.0
EDIT: looks like someone beat me to the punch. Regardless. the information is in that thread.