The Radeon HD 6670 + the A10's iGPU will probably be at best as powerful as a Radeon HD 7750. It is definitely less powerful than a Radeon HD 7770. I would skip crossfiring altogether because:
1. Only works with the Radeon HD 6xxx and HD 7xxx series and only up to the Radeon HD 6670 / HD 7670 (same card as the HD 6670, but only sold to OEMs)
2. The Radeon HD 8xxx is coming this year. Unknown if it will be compatible with Trinity APUs for crossfire.
3. Not all games will benefit from crossfire (or SLI). They must be designed to do so.
4. The Radeon HD 7770 is definitely more powerful.
Is the Radeon HD 7770 capable of playing every single game with high settings? That depends on the game itself and the resolution. See link below for some benchmarks; again the Radeon HD 6670 + the A10's iGPU is probably somewhat close to a Radeon HD 7750 in performance. http://www.techpowerup.com/reviews/Sapphire/HD_7770_Vapor-X/6.html