I'll add what others will probably second--if you want to just check it out first, a great try-before-you-buy option is Knoppix (
www.knoppix.net is one good place, or head direct to the source at
knopper.net.
You'll get a bootable Linux CD image which will go all the way to a graphical desktop. It'll run slowly because everything has to be streamed from a compressed CD image, but it'll at least get you an idea of what you're jumping into.
You CAN install knoppix direct from the CD, but you may want to go for a prepackaged distribution that's designed a bit more for that sort of thing. And there are far too many to choose from. Higher learning curve but more educational and really good: Debian; more corporate: Redhat; a bit more hobbyist but from what I've heard a very good distribution: Gentoo; new on the block but garnering a lot of interest: Ubuntu. I could go on for a while.
I'd say, start by booting Knoppix from CD. If you're still interested, be prepared to do a lot of research... but it's great; I only boot Windows for games, and even some of those I can run under Linux.
-->VPutz