Unfortunately this game doesn't scale with the number of cores at all, still the best unit is the one from the latest Intel Core2Duo, over-clocking at its best. That Am2 you're running isn't far off either, coming to answer your question, if you want to invest, increasing memory bandwidth will get you the most bang for the buck, replace your old GPU every now and then. But the increase in performance is rather lousy, considering the change of CPU including main-board will boost transfer rates by quite a margin. When it comes to this game you won't need an expensive rig, safe that money for your subscription.
so long Slpr