Check out this Application in the Mac App Store called boom, its $8.99. This provides a boost, what they claim of up to 400% in volume. It also provides a system wide equalizer.
I use this with my 17" and I can max my volume out with boom, and its about 3x louder and clearer. It doesn't drown itself out and it really sounds amazing!
My go's 13" unibody aluminum Macbook, uses this as well but the speakers do not sound as good as the 17" obviously, but from the inferior sound to begin with, it drastically improves it.
Well wrath $8.99, considering I use it on my iMac (for the system wide equalizer on my bose companion 5's) my macbook pro, and my mac pro.