Yes get a dual core cpu it's more future proof. And you can also get a x2 3800 if you can get it cheaper and then oc it to 4400 or even beyond. If you dont oc then get the 4400.
FX is just for bragging rights nothing special about it. Ever since conroe came out you cant even brag with the FX. You just look silly for paying that much money.
the fx 55 has a higher clock speed stock but the 4400 x2 is going to be more future proof.the speed difference can be made up by overclocking and when more programs are wriiten for multi core cpu's it will be faster