When I have my rinky-dink external speakers hooked up to my computer, even with the volume cranked all the way up it's not loud enough for my tastes. But there a trick you can use to increase the maximum volume that you get out of iTunes.

To increase the volume, open up the iTunes equalizer by going to Window -> Equalizer.

If you are not already using the equalizer, you'll want to check the box to turn it on. If you don't want your music to be equalized as a result, you can select the Flat option as the equalizer type. You can then crank up the Preamp level. By doing this, you will increase the maximum volume that you can get out of iTunes.