I purchased an additional 4870 at launch and was using them in crossfire, crashed non-stop but it turned out that it was one of the cards at fault. I use BOINC and during full load on the GPU's, even if they weren't in crossfire mode my system would crash. It wasn't an overheating issue just an old card not able to keep up, even underclocking would crash them. And since I like being able to play a game in windowed mode I didn't bother replacing the old one for a few more fps and a little higher rez.

Hopefully 2.0 can get a directx update for SLI and Crossfire users.