I dont think it matters which card you use in a crossfire setup.
Thanks a lot!
I found my answer here:
Can I run AMD Eyefinity technology with AMD CrossFire™ technology?
Any AMD Eyefinity technology configuration that works with a single graphics card will work with AMD CrossFire™ technology, however all monitors must connect to the primary graphics card. In most systems, this will be the GPU installed closest to the CPU. This is true for both AMD Radeon™ graphics and AMD FirePro™ professional graphics products.