Listen to Missy, you only want to gain functional muscle, if at all. If you just body build you will end up fighting guys who are naturally bigger than you which isn't a good thing, as Ricky Hatton found out recently.
I would recommend a high protein diet though, cause if you're training a lot you cause some damage to your muscles (even from running and punching a bag) which needs to be repaired. By doing a lot of boxing training such as shadowboxing skipping and bagwork etc., you are working the muscles which are relevant to a boxer which is the most important thing.

Doing some weight training may also be beneficial as being physically strong can also be advantageous in the ring, however out and out body building will give you dormant stiff muscles that constrict your movement, and make your muscles prone to tears.