I would say no, but you will have jackasses such as the above come in and argue tooth and nail, throwing out fanboy left and right. I believe I heard that the 7850 is a pretty good overclocker, so combining two of them can give you pretty good performance. However, some of the 670's also OC pretty well, many get them running around 680 levels, and the concern with crossfire is micro stuttering can occur. Your call though, or maybe someone more familiar with AMD can chime in.
I am not biased to either, I just have heard the positives and negatives between both setups, and generally I came to see most feel it is best to go with a single card setup if possible.