Which 7950 to run Crossfire with in the future?

The only issue with crossfire is the first card may overheat. That is, having two double slot cards right next to each other may cause the one that had the second card above it to overheat since the area above it where it gets its cooling air is taken by another card. To fix this you need a case with good cooling, preferably one with a fan on the side right above the Radeon farm.