The Crosshair IV Formula is the original Crosshair IV board without the Hydra chip. The Crosshair IV Extreme is pretty much a C4F but with Hydra and RoG Connect. About Hydra, it's completely different to CrossFireX. The HD 5850 does not clock down AFAIK, and your HD 5770 becomes like a booster GPU, just to add some FPS but not really CrossFireX. That's why you can run say a GeForce GTS 250 and HD 4850 together, which in some games can provide over 75% boost.