I would like to second the recommendation that someone made to you in another thread. Please pick up and read Nancy Clark's sports nutrition book.

ALL of what your body needs pre-, and post-workout (and much of what you need *during* a workout) can be obtained from real food. And for alot less money than powders, mixes and bars.

I'm not saying that these things don't occasionally have their place (and I'm a huge fan of Gu Roctane while running for it's ease of use and digestion) but generally speaking, she believes, and I concur, that you should get your nutrition from real food.