Having said everything there is to be said in this thread by various people, I would like to add one last thing:
It is not uncommon for beta software to run more slowly than the final version, because the code must log everything it does and that creates overhead. So maybe the situation won't be that bad after all once WotlK goes gold.
My advise is to wait and see how it runs in your machine with the configuration you want, and then make any decisions for upgrades.