Hmmm.. I can't really say here. The games you are playing will most likely benefit from a quad-core system, but then again, the Athlon IIs drop the L3 cache.
Also your cores probably aren't unlocking because they have been binned out.
If I were you, I'd decide whether I really needed those two extra cores. Do you really NEED a couple frames per second more, or can you live with a dual core?