Migration Assistant is pretty smart... and the Assistant part will walk you through most steps. Take a look at it first.
Old support page
here
One caveat may be if your 'old iMac' is PPC. There are
some apps that don't migrate to intel (you'll need to update to the Universal Binary).
Note also that newer iApps may rewrite the library format and thereafter not be usable with older software. iTunes and iPhoto, for example, update the old libraries to support new features. The data will survive, but if you're worried about reversing the upgrade path in the worst case, you should back up your data first.