FX = the best. otherwise it wouldn't be named FX.
but, with a little elbow grease (for the cooling part at least), some patience, and some knowledge about overclocking, one can come dangerously close to FX speeds.
i don't know if any of you picked up on it, but all of AMD's architectures are the same, since they stopped producing Athlon XP's.
the only difference EVER is cache size, thats what sets the sempron's apart, and the fact that they dont support socket 939.
basically, opteron's are underclocked FX chips (or FX chips are overclocked opterons) and athlon 64's have a little less overclockability... for whatever reason.
either way, they all perform just as good... and when i say opterons are the same as FX's, i only mean they're similar.
the FX's simply guarantee a certain speed
if i were building a new computer, i'd go with the cheapest (dual core) opteron.
i'm waiting at least 2 years before i do any big upgrading though.