I recommend trying Garnome
. It installs to a directory tree of its own and will not interfere with other Gnome installations. If you want to remove it, simply delete the directory and remove the entry X session entry and you're done!
It defaults to installing to ~/garnome but I prefer putting it in /usr/local/garnome (or /opt/garnome) to make it accessible by all users. If you add the LD_LIBRARY_PATH entry and related ones like the PATH, as described on the homepage, to your other X session types you can benefit from the Garnome libraries and apps no matter if you run Garnome or not. I usually use XFCE4 or Fluxbox but I like having up-to-date libraries to build apps against and Garnome is ideal for that too.
The current version of Garnome provides Gnome 2.5.92, which is a developer's preview of Gnome 2.6. A "true" 2.6 release from Garnome is expected very soon.