Before replacing your bottom bracket with a new one, I'd remove the crank and check the bearings by hand. If they feel smooth, remove the cups, grease the threads, then torque to spec. Do the same with the crank arms, installing per Shimano's instructions. If the creak remains, there are a number of other causes for creaks in addition to bottom brackets. Visit http://www.sheldonbrown.com/creaks.html to learn more.