There are plenty of programs that will assist in syncing iTunes to regular MP3 players, since most mount as a removable drive. Most will sync certain playlists, though if you want the whole library, and if you're cheap, you can probably get a free backup tool that will sync your whole iTunes file tree to your external card. Each time you add songs, just run an incremental backup to your card.
I personally use Missing Sync (for OS X), but I googled and found this
http://www.pocketmac.net/products/ppctunes/
Another link for the Mac guys
Sync Playlist Files to Folder
http://www.dougscripts.com/itunes/index.php