You can install the Xfce Package over your Gnome desktop, then choose which to 'open' at the Log-in.
If you decide you don't like Xfce ... then never click into the Xfce session/desktop again. It won't harm anything, if it stays there.
I've done this before, and have never experienced any problems.
You don't have to 'remove' the Xfce Package is you don't like it ... just leave it alone.
Your original Gnome desktop, will not be changed ... everything in Gnome remains the same.BUT ... you will love the Xfce Desktop Environment
so much better than Gnome. Everybody does.
Checkout some of the other Linux Distros with Xfce as default. While you're waiting for Mint-9 Xfce CE to be released.
I have downloaded, installed and tested every Distro that uses Xfce, over my time in Linux ... Mint is one of the best.