Assuming you let iTunes organise your music files (check the Advanced section of the iTunes Preferences) then it should be a matter of making a copy of a folder named simply iTunes in your Music (or My Music on a PC) folder. Within this should lie the song files and the relevant data files to maintain playlists and play count information. 🙂
Remember to de-authorise any iTMS purchased songs too.
Once you get the new machine, tell iTunes to keep your files organised again and then put the iTunes folder back into the equivalent spot on the new machine and reopen iTunes. 🙂