That's a decent article on boxing basics. I would recommend that you check out www.crossfit.com for building a cross-functional foundation of power, strength, and aerobic and anaerobic endurance.
Full-contact twists (http://www.bodybuilding.com/fun/mm6.htm) and other core rotation movements, as well as exercises that develop explosive upper-body power (e.g., clean and jerk, snatch, push press), would also be rewarding to work with.
