I tried running two 6870s in Crossfire for a while. Any game that I throw at it runs just fine, although GTA IV does get the fans turning a bit. Running a gpu client on each card at the same time though, is extremely painful. The top card overheats and reaches temps of ~108C before it clocks itself down. The downclocking then decreases temperature, which lowers the fan speed from 100% to 67%, which increases the temperature, which increases the fan speed, which clocks the card down again. It's horrible and definitely not healthy for the card.
Ok, a bit of rant. Anyway, any single card should be stable. Be careful when you run more than one.
had same problem (have micro atx mobo and case). installed water cooling system - no problems so far )