I have a Video iPod and a Rio Carbon 5 gig and a Windows Mobile MP3 phone, and I use the same music files for all of them. With a little practice, you'll find a way to synchronize everything perfectly.

If you rip music files from CDs or buy them off the Internet, the files have all the album, artist, and track info in the MP3 file. However, if you steal music off the internet, it often comes without this info.

If your iTunes isn't detecting all your music, just manually add it by folder. I think it's [File -> Add Folder] or something. I store all my music in the "My Music" folder, and so it's easy to add it all to iTunes just by selecting the top-level folder. If you have any WMA music, iTunes will convert it to MP4. I suggest afterwards, that you delete whatever WMA originals you have and replace them with the relocated MP4s. This will make your library "universal".