CrossFire motherboard click here
You will nees the correct motherboard to enable Crossfire.
If you have an NVidia chipset SLI motherboard you can use either ATI or NVidia cards to run with a single graphics card, but you will need NVidia cards to set up a dual card SLI rig.
Just a thought.....
Aiming to build a high end rig as a first build project might in itself be an expensive mistake.......what is the complete spec of the pc you are considering ?