since you have the mic already, why don't you try sticking it in front of the amp and see how you like the results. Try a few different positions close to the speaker cone, closer to the edge of the speaker etc.
There's nothing to say you have to use a dynamic to record an electric guitar.
SM57 are good for very loud amps because they can handle very high sound pressure, often more than a more delicate diaphragm in a condenser mic could handle. However if you are recording at sensible volumes (less than that of a jet engine from a inch away say) then a condenser will work fine and usually will give you more detail and top end than a dynamic.
IMO/IME it's usually better to try with what you have before just arbitrarily dumping money into something else. Maybe what you already have will give you great results at no cost to you.