When migrating old computer what should I keep?

Here's a link that should help you get started:
http://support.apple.com/kb/HT5872

 

Heh, that tells you everything other that 'what you should migrate'.

Upgrade the existing iMac to Mavericks if you haven't already done so.

Personally I'd be inclined to migrate user accounts only, and go for clean installs of all apps. Takes longer but weeds out the junk. There's also the option to import 'other data' but finding out exactly what that includes (beyond various universal OS preferences) is next to impossible.

 

I don't think there's any benefit to a manual transfer.

Apple seem quite confident that the Mavericks Migration Assistant can handle transfers from 10.6.8 and above...
http://support.apple.com/kb/HT5872
...which (if true) means that there's no need to update the OS X on your old Mac.

Migrating a user or users will transfer everything that pertains to those user/users, ie the contents of the named user folder: Desktop, Documents, Downloads, Movies, Music, Pictures etc. It also transfers the user's Library which contains all your browser information, your Mail, and all user-specific preferences for both the operating system and for individual applications.

I was being rather ambiguous when I mentioned 'other data' earlier. There should be a folder named 'Shared' within the Users folder. If this has any content you wish to keep then in addition to migrating named user you'll also want to select the 'Other files and folders' option in Migration Assistant.

The 'other data' I was warning against is actually named 'computer and network settings'. This is where the junk resides; if you have the faintest suspicion that your current system is 'wobbly' then do *not* migrate these settings, because you're very likely to be migrating the problems.