Maybe don't buy any just yet, unless you have no other means of obtaining the discs and also need manuals+support. Download, if you've got broadband, or get off of magazine covers, or buy from ebay or magazines for £2-3 LIVE CD's of lots of distros eg (if available):
You can boot the LIVE CD and see what they look/feel like without making any changes to your current system, then if you like one or two, try getting copies (again download from net or get from ebay or similar) then try them for real, then If your really happy with it, buy a full version.
if you prefer to have chunky manuals and a bit of support, buy the boxed ed straight away eg
Not sure which others have chunky manuals.
Most distros allow you to re-partition your drive, so you can dual boot linux + winxp, but it's probably worth getting a couple of LIVE CD's so you can see and get an idea without doing any damage...
I think, therefore I compile