Will a graphics card boost performance?

Right now I have an ATI Radeon ASUS (I don't know whats what there so I thought I'd include it all) 5770 graphics card. Thats all Iknow off the top of my head. I bought it becasue it was in my price range and looked decent compared to similar priced cards. Also my whole computer's pretty much ASUS.
It has crossfire X and I was wondering if adding another would make my computer more bad arse. According to what I know it will run fine with everything I have. I just want to know if having two graphics cards will make my computer faster or more efficient in any way.
Thanks from the noob.