How do I back up my apps without the need of re-downloading from iTunes when I restore them?

I have an iPod Touch(2nd) and all the apps I have left on it still works cause I've never updated them. I know you might say I should to protect... blah, blah, blah. I only use it as an MP3 player, but I sometimes use the apps that are still working.

Is there anyway for me to backup the apps with their old firmware? Forget iCloud if you were thinking that. It's a 4.2.1 firmware iOS. I don't want to end up with findout that the apps are the new version requiring my iPod to have the latest firmware to work.

Thanks.