Welcome to the forums. Have you tried any distributions yet? If so, what have you tried and what have your thoughts been with where they had satisfied you and didn't. Also, do you have any preference on the desktop environment (e.g. KDE, GNOME, Enlightenment, Fluxbox, etc...)?
Ubuntu is always an easy choice for new Linux beginners. Fedora is also very nice, and is the distribution I personally side with. If you haven't checked it out before, DistroWatch (www.distrowatch.com) is a great site for information.
The learning curve for Gentoo may be a bit too much, but with their recent LiveCD graphical installer and the "Seeds" project, it may work out for you.
I would recommend just trying out various distributions till you find your perfect mate. Most of them offer LiveCDs, so you can simply boot from that without messing with the data on your hard drive (For Fedora the LiveCD is the Kadischi Project).
Yes, you can have multiple desktop environments installed and switch between them.
I highly recommend Gentoo... but not until you use, and learn well, another distro first. Michael had two great recommendations with Ubuntu and Fedora. I try to stay away from SuSE though... it's one distro that seems to be getting buggier with each release.
Gentoo is too unstable for me. If I have to go off somewere and don't get a chance to update my desktop system for a couple months (not a insignificant task) then it's difficult to deal with the breakages that occure.
But that was a long while ago since I tried Gentoo. Maybe it's gotten better. The realy major thing is that when I want to install a new program I realy hate to have to wait for it to compile and compile it's dependancies before I can install it.
Personally I prefer Debian, but it's not very newbie friendly.
Choosing a distro is like choosing a freind in a strange sort of way. Each distro has a different 'personality' a different goal or purpose.
This is all that realy sets them appart. Since it's all open source then all of them have about the same hardware support, the same software support. Everybody generally uses about the same code as other distros.
The differences are in things like how they setup the 'init' system, package management system, default install (a big one), and the amount of software the distro supports directly. A important distinction is also community. Some distros are very friendly, while others aren't. Some are desktop-specific with focus on multimedia and such, while others want to be minimalist and fast on a older hardware, and others want to be general purpose.. you can use them just as easily for a server as a desktop.
A big one is the packaging system. In Linux you generally want to install your software via packages if you can help it. Going to mozilla.com and installing firefox directly isn't realy the sort of thing you do. You install firefox via the package management system. Using installers is usually for propriatory software and games.
Ubuntu is a good one to start off with. It has a nice default install. Lots of software aviable in add-on repositories (were they store packages online). A healthy community with helpfull forums and good wiki-based documentation for common problems (such as setting up propriatory 3d drivers, or getting wifi cards to work). It is also usefull and good for both very new users and very advanced users.
Be sure to read the documentation and join the forums. Chances are if you run into a problem a dozen people have already been-there-done-that and talked about it in the forums and if they found a good solution they would document it in FAQs pages or wiki pages.
Also it is very important to make sure that if your following along with documentation that it is the same version as the system your using. Ubuntu releases a new operating system every 6 or 8 months or so. It's not like Windows were something written 3 years ago will be accurate today (since XP is about 5-6 years old now)
After that you settle in and if you like it you can stick with it or try out a few different distros. Most people will try 2-3 or more before settling on one they like and even then they will try out different ones every once in a while.
Some people like ones that cater towards advanced configuration and tweaking (such as Gentoo) others like smaller communities and such. Others just like sticking with the very popular distros as software support is generally easier to deal with. It is just about what best suits you or what you want to do with your computer.