I am running Emacs 23 on Ubuntu 12.10. I see that there is a package called emacs24; will installing this package overwrite my current installation?
It doesn't. I also have Emacs 23 installed on Ubuntu 12.10 and just installed Emacs 24 from the Ubuntu Software Center. The two app versions run side by side independent of each other (although they do share the .emacs file among other default paths, etc.).
You would have to uninstall Emacs 23 separately if you wanted to just have one installed version of Emacs.