I run MSI 6950 twin frozr III in CFX atm. The heat that the 2 cards generate is pretty nuts. The top card gets choked out due to the fact the card in the second slot is so close not allowing good air flow to the card.
It's pretty typical for the top card to hit high 70's or low 80's (if its warm out). It has me looking into liquid cooling.
There is the problem. No one makes a full block to cool the cards. So I would recommend that you choose a differant card that has that option. Possibly a Sapphire Toxic
Cooling this set up with air, while doing anything that even remotely taxes these cards is going to be very loud and very warm. Maybe that doesnt bother you and being able to compare your desktop to the deck of an aircraft carrier is amusing. But, having the option to use full blocks to eliminate those issues from the beginning is my best advice.
GL, post some pics (build and performance) I am considering this as well next year.