The only FX chip for AM2 as of right now is the FX62, all the rest of the FX series are 939.
AM2 takes DDR2.
An FX-55 is a socket 939 not an AM2 socket. There is different number of pins in a different cnfiguration. You won't be able to use this rocessor in an AM2 mobo, you need an FX-62 to work in a socket AM2 mobo.