heres the general rule of thumb for added performance with added cards, if your talking games that scale up faily well. in some cases a single card outperforms multiple cards.
adding a second card boost performance by about 50%
adding a third boosts an additional 10-30% many games get little to no performance gain from the third.
adding a 4th boosts by less than 10% most games are not helped at all by the 4th.
Idealy youd see +100%, +50%, and +33% for the 4th. so you are far from getting your moneys worth with crossfire or SLI