mate, if you want to build it up theres many ways
good old fashioned bench press is the daddy. however, muscles increase in size and mass faster when you keep changing the way you train them. its called tricking your muscles. train them one way untill your gains slow down, and move onto a nother method of training, and youll make gains again!

the core excersizes are

flat benchpress (does the whole chest, but with a spread out effect)
incline benchpress (where your sitting more upwright as you bench press. this builds up the upper chest)
decline bench press (where your leaning back when benchpressing, this does the lower chest)
remember guys, when you bench it doesnt have to be a bar, infact swapping to a dumbell in each hand is a good way to trick those muscles and get them growing again

pressups are the daddy for conditioning and endurance training, and your sure to see *some* gains
dumbell flyes are a good one, personally i really feel this working at the moment, but im trying to cut down on the weights cause im WAAAAY to heavy to be fighting in my needed catagory
dips while leaning forward are like a hardcore pessup, youll be surprised how much you ache afterwards!

recently while in the gym a hufge bloke showed me a muscle "tricking move". grab a heavyish dumbell and lay on a flat bench. grab the dumbell by one of the weights, with your arms straight above your body, like you had just completed a rep of bench press. keeping your arms straight bring your arms back so theyr in line with your head (like you were doing the Y in YMCA ), and return it to the starting position.
this absoloutely kills, and i can feel it working the upper abs, which co-incidentally is a hard area to work.

i would warn you mate that gaining alot of weight in boxing perhaps in the greatest idea. sure youll look nice on the beach, but youll have to fight the big boys. what good is a set of pecks when you look like spongebob from all the smashing youv taken from the heavy hitters