iPod Compressing iTunes library for export?

[arogge]

I'm interested in compressing my iTunes library of MP3 files for export. How do I tell iTunes to convert all MP3 files to something like 64 kbps and export the compressed files without altering the original files? Either that, or maybe Apple would like to come out with that new iPod Classic with a much larger capacity. Until that happens, my iTunes library will not fit on any of these flash memory devices, so I'd like to compress my MP3 files.