Nitpick: it wouldn't be a potential 216. Some three-"letter" sequences code for the same amino acids, so instead of 4^3 (64) possible amino acids, only 20 are generated. Adding new letters doesn't change what these old words create, so I think there would only be a possible maximum of 172.

(I think I did my math right, but maybe not.)

(edit: thanks duaneb, had my basic bio facts wrong - codons code for amino acids, not proteins.)