The balance issue for GM in my opinion isn't so much overpower but information overload. You know what your Player characters have and there stats, if you have a brain you've got a copy of their character sheets and current goodies. All you need to do is remember the basics, No one wants to hear someone else's soliloquy especially if it doesn't advance the story, no one wants to be the arch mage's wand carrier or the great knights shield bearer. You are in essence and with the help of your players creating a story and everyone needs to have a part and that part must be important enough to keep them interested and active. And the mission they are about to embark upon has to be challenging without being impossible and that means you need to understand your players and their capabilities as well as you do the game. and the characters they're playing. GM is a challenging roll and it isn't for everyone.
Oh and for what it's worth I am currently replaying Hordes of the Underdark, My current character build is a Human Level 4 fighter, level six thief level seven weapon master specializing in Short sword. In about two more levels he will be duel weilding Enserric the shortsword and the sword of quickness.