With a cracked screen, though, your current iPod isn't likely to be worth much. You can look on eBay to see what other iPods of the same model have sold, but depending on what new iPod you choose, the 10% discount may well be more, or at least close enough to make it not worth the hassle of trying to sell your damaged iPod.
If you want to try and improve your current iPod's performance, try the usual steps: restart, reset, restore.
If you try restoring from a backup and that doesn't fix the problem, try restoring to factory settings and synching your apps. You'll lose the app data and settings, but if the problem is due to a corrupt cache or settings file, that should cure it.