The OL 2007 install is a separate download and I don't think you have
to install Office 2007. However, if you install it on a machine with
an earlier version of OL, my understanding is that you can't go back to
the older version. I guess you could backup your data, install OL2007
and if you didn't want to keep using it, uninstall it, reinstall the
older OL, and restore your data. Given the warnings on the Office 2007
site about OL, though I'd double check this approach with MS Tech
Support before proceeding.
Ken