It is unknown to most that the FX-51 and early models of the FX-53, despite belonging to the godly FX family, originated on socket 940 package before the uniform 939 platform was introduced. In other words, the FX-51 has to deal with the expensive features that come along socket 940, in particular registered memory, and a limited selection of good consumer boards, so as I mentioned, you paid a boatload for the FX-51 to get a lot of features you really don't need and could have gotten a similar performing system for much cheaper
But as I said, it's not a bad system by any means, but you don't really want to upgrade it and if you are you would be better going with a complete overhaul IMO