Multiplier is the main difference, the cores are essentially the same
(except for the Opterons in the 2xx/4xx multiprocessor series) and hence
performance is identical.
Athlon64 allow multiplier change downwards but not upwards.
Athlon64 FX allows multiplier changes both upwards and downwards (thats
why overclockers with lots of money to burn prefer them).
Opterons don't allow you to change multiplier at all (ie most restictive
for overclocking).
So if your not planning to overclock get the cheapest one at the sped
your want (almost certainly its a Athlon64 although there are some cheap
1xx Opterons out there).