If you did want to backup the apps, they're already stored on your PC as the filename and folder shown in iTunes when you select, Library- Applications, then right-clicking on it and choosing 'Get Info', in the 'Where' part shown in the 'Summary' tab. Those files are updated to or from your iTouch to the latest version when you Sync it if you have updated the app either on the device or in iTunes.
If you are wary about updating to the newest version of a particular app (in case the author has added advertisments or otherwise ruined what was a perfectly good app, and want to go back to an older version), you can make a copy of any or all of those .ipa files elsewhere, and after removing the app from iTunes and your iTouch, simply double-click the older backed-up .ipa file and that older version will be re-added to iTunes and it can then be synced and added to your iTouch as that older version. Note that those .ipa files are DRM protected and will only work if the account and device have permission to use them, so it is not a way to give copies of apps to friends.
