Install one of either Fedora 11, Ubuntu 9.04 or LinuxMint 7 on your most powerful (storage/memory/processor) computer. THen install VirtualBox-ose (google it) on that computer. After you have VB going, download install and/or LiveCD editions of as many distributions as you want. Install these distro's in VB and test them out.
This method is less involved and difficult than trying out different LiveCDs everyday. It has the advantage that you can directly compare different distros as you run them side by side on in VB. At them same time you are gaining experience running your main distro.
As a long time Linux user (Ubuntu) I'm using this method to help me switch from Gnome to Enlightenment DR17 (e17). I'm staying on Ubuntu 9.04 for the time being so I'm running e17 on it in VB. This means that I don't disrupt my daily computer use wile learning and configuring e17 to my satisfaction.
hope this helps.
P.S Head over to distrowatch.com to find distros. There is a hierachy and a sequence to adopting distros. I would only start a newbie with, Ubuntu, Fedora, Mint, PCLinuxOS, MEPIS or Mandriva. The rest you graduate to.