It'll remove the content from the iPod touch. If you want to reduce the amount of content which gets deleted, use the Transfer Purchases function in iTunes before syncing it, and if you want to preserve the application data, perform a backup and restore.
Nothing will be deleted from the iTunes library, but it should be backed up anyway.
- It depends. If you follow these instructions you can make that computer the syncing computer and not lose anything.
- If you do not want to change syncing computer but only want to load music from the computer to the iPod use the Manual Method of syncing specified here: