Athlon X2 vs Athlon 64 X2

The official family name is "Athlon 64 X2"

But you may see some people use the following terms:

X2
Athlon X2
A64 X2

or just the model numbers
3800+
4200+
4400+
4800+

With the exception of "3800+" those model numbers only apply to dual core Athlon 64 X2 CPUs

3800+ comes in both single and dual core flavors.

They all mean the same thing "Athlon 64 X2".

The Athlon 64 "FX series" is similar to the regular Athlon 64, but they are fully unlocked. Regular Athlon 64s are half locked meaning the clock multiplier can be set lower than the default. That done so Cool 'N Quiet will run. The Athlon FX are fully unlocked, so that you can overclock the FX simply by increasing the clock multiplier. The Athlon FX-60 is a dual core CPU, lower FX model numbers are single core CPUs.

There are three different Athlon 64 and FX sockets; 754, 939, and 940. None are compatible with each other. Socket 754 was used by the first generation of Athlon 64 CPUs and by current Sempron CPU. Socket 939 came later in the "2nd generation" Athlon 64, dual core Athlon 64 X2 are socket 939. Socket 940 is meant for servers not for the typical home user.