Gate9er :
You cannot crossfire a cpu with a graphics card. Crossfire X is a dual card configuration.
Yes, you can "crossfire" the 6670 and the 5400k. For the sake of semantics, it's called dual graphics when you're pairing a discrete card with an APU.
You can save some money by buying a 6450 instead of the 6670. The 6670 is more powerful than the APU's graphics and it's performance will be throttled down to the APU's level when you pair them.
Microstutter has been pretty much solved. Turn off v-sync.
http://videopump.net/video/0zRoIH3dTFI/amd-a6-5400k-apu-in-dual-graphics-with-the-amd-radeon-hd-6450-review/