FYI, you should usuall wait more than 8 minutes before bumping your post in such a manner.
All the support on these forms comes from users donating there spare time when it is possible.
To answer your question then, the ASUS M5A97does support CrossFire, but only in 16x/4x mode.
As such, your performance with higher end cards will be quite bottlenecked.
With a pair of 6670's however, you should be in pretty good shape.
On a side note, unless you already have a pair of 6670's, you would be much better off with a single, more powerful, GPU.