have you thought about the fact that these cards consume 200-300W and that to have 5 of them your PSU may need to be monstrous?

Also seems this only makes sense now if you want o resell or use those cards elsewhere .. if buying only for Bitcoins some cheaper cards may make more sense based on how much processing power you get per card cost. 

7970 - 600MHash/s and $400
7770 - 200MHash/s and $100