To answer that question well requires questions for you:
- What do you use your computer for, organized from most to least?
- Are you genuinely interested in computers, learning how they work, and learning new things? Linux is so utterly different than Windows that many find that getting used to one and trying the other is an alienating experience.
- Are you content with your current setup?
- Do you have plenty of free time and patience?
- Are you sincerely interested in having a great deal of power over your computer, rather than being able to do just what a particular vendor thinks you want to do? Computers are capable of much more than Windows is capable of (or any OS for that matter)
- Are you interested in programming? Linux is a better experience if you know C.
As far as the best distribution of Linux-- What do you value? Some distributions are easier (Mandrake, Lindows, Lycoris) some have superior systems of software installation (Gentoo, Sourceror) some are blazing fast from the get go (Gentoo, others) some are particularly popular and well supported (Redhat, Mandrake), some are outstandingly easy to install (SuSE, Mandrake, Lycoris, NOT Gentoo).
For a brief look at a large numbre of distros, you might be interested in
Distrowatch.com, but that site is of limited real use. Everyone has a favorite distro. It might be best, if you have the time, to try several and pick whatever works best for you. Good luck--you are about to exit the world of beginner operating systems.