In my opinion I don't think you should get a FX processor. Not only might it not be compatible with your motherboard socket, but they're a lot slower than the phenom II in terms of their architecture. You could order a hexacore phenom II for about $150 from microcenter, or look on ebay for a better price.
Edit: Looked up your motherboard, you would have to replace it completely if you wanted to use anything other than an Anthlon II, Phenom II, or Sempron 100 processor (Phenom II is the certain best of those)