The best is to copy iTunes library from the computer where your iPod used to sync with.
Refer to these articles:
iTunes: How to move your music to a new computer
iTunes: Back up your iTunes library by copying to an external hard drive
If for any reason, the old library is no long accessible, refer here:
Sync is only oneway, from PC to your device. Unless you have the music on your PC, iTunes is going to wipe out what you have on your device if you are syncing to a new library.
You can only transfer Purchased music over to Itunes on your PC.
iTunes Store: Transferring purchases from your iOS device or iPod to a computer
As for you own music, you may have to use a third party software. A good Free one is called Sharepod which you can download from Download.com here:
Anytime you sync to a new library, itunes is going to wipe out everything from your devices. The idea is have all the music in your new iTunes so that all the music will be synced back to your device.
Transfer of purchased music and copy over your own music and add to the library will do the job.
Yes. itunes will remove them all from iPod then sync back what you have in your iTunes library. Therefore it is important that you have copied all the music and they all appear in your iTunes library.