It's definitely possible. I installed Gnome on my computers (was always a KDE guy, then switched to XFCE, then wanted to try something new). I didn't really like it, though - it's just not my style - and installed KDE (which I never really used to like under Mandrake, but it looks GREAT under SuSE!).
The one thing you might want to keep your eyes on is the "My Computer" icon. KDE needs it to point to "media:/", while Gnome needs (correct me if I'm wrong) "drives:/".
So if you click on the icon and nothing happens, don't worry, you haven't messed anything up!
By the way, don't stop with just Gnome and KDE. Give XFCE a shot, maybe Blackbox, and a few others. It's fun to give your system a makeover now and then.
The best thing is, playing around with new desktop managers/environments won't commit you to anything - you don't like it? Fine, just choose a different on on your next login!