Might need to make sure you will have proper cooling, especially if they are going all on the same mobo.  Mine are in an open environment so are not that bad, but I have a fan on my 5970 cause that thing will get LOUD otherwise. 
I was planning on running 4 in mine, but I feel best that I just run 3.  Even with coolers on them the middles ones just couldn't get enough airflow. 


Did you pick up those Visiontek ones for uber cheap?  I thought about buying 1 or 2 of those but I don't dig reference coolers anymore so opted out. :-/



When you game, if you game, just enable crossfire when you game.  With those cards, you don't even really need to crossfire unless you are running eyefinity.