Make sure you copy the actual files (residing in your iTunes library), as opposed to burning audio CDs of them, because burning audio CDs changes the format and would result in a loss of quality if you ripped the music back to the computer. Whether you use networking or data-format CDs or DVDs, you simply want to move the files you downloaded from the iTunes Music Store to another computer, just as you'd move your word-processing files or photo collection.
Deauthorizing/authorizing the old/new computer is what makes the files unusable/usable on the old/new computer.