It does seem like with the Opterons not OC'ing like they were, there isn't a whole lot going for them any more.
At Newegg an x2 3800 is $325
An Opteron 170 is $420
Both are dual cores at 2.0GHz each.
The Opteron has double the cache.
Even with double the cache, the Opteron is going to need to be a pretty good OC'er to make the near $100 bigger price tag worth it.