Its the Athlon 64
FX, not Athlon FX. The Athlon 64 FX series are basically high-end (2.4-3.0GHz) Athlon 64 CPUs. The only thing different is that they have their multipliers unlocked, unlike regular Athlon 64s who are "top locked." They just gave them the "FX" name because they wanted to distinguish them, and the name sounds kinda cool.
Athlon 64 X2 Dual Core CPUs are like simply having 2 single-core Athlon 64s in one PC. Your clock speed won't double. You'll simply have two cores to use, instead of one. Its the same principle as having two different processors on one motherboard, except this is better. The benefits? As far as gaming goes, only a handful of games are even STARTING to support Dual-Core CPUs so basically, you'll only see the performance of one core in games. As far as other tasks go, however, multitasking will become incredibly smooth. For example; you'll be able to run 4 games at a time
, something you couldn't even dream of on a regular single-core.
Incase you haven't noticed yet; AMD DOES have Dual Core CPUs. What more, they blow away Intel's Dual Core. There is NOTHING that Intel Pentoim-Ds excel in compared to Athlon 64 X2s. There are a couple of reasons for this, including that the Athlon 64 X2s don't have to go to the Northbridge to talk to eachother, and that they aeren't starved for memory bandwidth like Pentium-Ds.
HyperThreading is like fake Dual Core. Yes, it ends up having the same effect as Dual Core, but not quite to the same extent. HT is more like 1.5 cores actually, ask Intel themselves. Basically, no one comes close to the computing power of the Athlon 64 X2s.
Can you afford to buy a new processor in the future? If so, do as chickenfoot said and buy a Athlon 64 Single-Core, then upgrade to a Dual-Core in the future (yes, Socket 939 supports them both). If not, then how much CAN you spend right now? Depending on that, go with as high of a Dual-Core CPU as you can.
Bleh, one more thing I guess I should mention; Intel Pentium-Ds are cheaper than Athlon 64 X2s, but like I said, they're no match for the Athlon 64s.