iTunes can maintain multiple formats of the same song in the same library. Start by importing the music uncompressed (wav of aiff format). Then go to Preferences and change the importing format to your desired compression scheme (mp3 or aac). Highlight the songs you want to format change and choose Advance/Convert Selection to. iTunes will keep the original uncompressed files unchanged and generate new files in the compressed format. You can then use the smart playlist feature to easily sort the two different formats.
BTW, Pableson is right. You won't be able to fit all your songs on the iPod regardless of the compression scheme you use.
BTW, Pableson is right. You won't be able to fit all your songs on the iPod regardless of the compression scheme you use.