I don't know AMD stuff back and front, so just google info about that board to find out. Unfortunately I think it's quite likely that the old SK8N can't support any dual-core CPUs such as the Italy I mentioned before. But it'd be worth it to try to find out.
So I guess you're pretty much stuck because as far as single cores go, you can't really upgrade from the FX-51. I think you could get another 200MHz from a top dog socket 940 Opteron, but that's not worth buying a new chip for.