http://www.cpu-world.com/news_2010/2010102...able_cores.html
There is a new generation of Phenom II's coming out, including a Phenom II x4 3.5GHz with potentially unlockable cores, turning it into an x6 3.5 GHz. I would look into that, and other existing processors with unlockable cores, before spending $180 on a 2.8 GHz 6-core. Just make sure you pay attention to part numbers if you are going to be saving money that way, and read every comment of feedback if you decide to get a CPU with the intention of unlocking it.
BTW, I already looked, and they are not in stock on Newegg yet. But there are other AM3 CPUs with unlockable cores that are in stock. They are mostly lower end than what you seem to be aiming for though.
I also agree with others saying it's pretty much a waste to crossfire those cards, when getting a single higher end card will do you nicely. You might also be setting yourself up for a lot of avoidable frustration doing a crossfire setup on your first full self-build. I'm not saying it's any harder-- but for instance, if one of the cards is DOA, or if you have other issues, it could take you a whole lot longer to get from "point of sale" to "dream machine" within your budget.
(As opposed to getting a single card, which is a setup you've done before, and you'll be able to figure out what, if anything, is wrong, a lot more easily.)
I just think that you'd save money and/or headaches by setting yourself up to do a Crossfire and/or high end/unlockable-core processor
later on when Crossfire is more broadly utilized; and getting a less expensive CPU now because of the newer, better ones on the way within months.