GPU Cafe claims a Radeon HD 4670 CrossFire setup is a tad faster in 3DMark benchmarks than a Radeon HD 4850.
From these preliminary benchmarks, the 4670CF seems to have a slight edge over the 4850 and we'll have to wait and see how this translates into games. Judging from the raw specs, these two might trade blows as the 4850 will have a clear memory bandwidth advantage while the 4670CF setup will have a clear advantage in texturing power.
Interesting, but it will probably be better to get a Radeon HD 4850. Official info isn't available yet but I'm guessing two Radeon HD 4670 cards will be a bit more expensive than a Radeon HD 4850, use a bit more power and then there's also the fact that not all games support CrossFire.