iTunes has a feature that allows you to import and store CDs in lossless format, and convert to a more space efficient format when syncing to you iPod. This way you can maintain a bit perfect copy of your CDs for listening at home and future use, and choose the format (128, 192, 256 kbs) so all your music fits on you device.
Once you've imported a few CDs and sync'd to your iPod, you can do the math to extrapolate the space requirements for the rest of your library.