Hey, cool that you're going to get a crossfire setup ( I have 2 xfx 6870's ). It is indeed very easy to setup crossfire, you place the 2nd card into your mobo, get the connector on and boot up. In Cataclyst there is a simple option where you can turn on crossfire. Regarding the CAPS, every once in a while they release these ( check
http://sites.amd.com/us/game/downloa...-profiles.aspx on a weekly base ), you uninstall your old one and install the new ones, they are always ~0.5MB and contain new updates + the previous. So all you have to do is keep dloading the new one while uninstalling your old one and you'll get the most out of your crossfire performance
Good luck!
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Small edit ; missed the why part ; they contain updates to increase the game performance (don't ask me how they do it, but you will see an FPS increase :P )