the way you are going to have your partitioning set up (20 for linux, 100 for storage and fappage material, 40 for xp) you won't be able to have access to the 100 from both operating systems. (as far as i know, they use different file systems). In my opinion, i would keep the 100 for storage with your xp.
As for booting. You'll want to install xp first and then install linux on the remaining partition. If i were you, i would start off with either fedora (redhat) or mandrake. Both of these distro have an excellent boot loader at start up. You can choose which O/S you want to boot into or they give you 30 some odd seconds and then boot into Linux. This is the EASY way to do it. Installing xp after linux will cause you unneeded problems since you are so new to linux.
When choosing a distro, what you really want is simplicity during installation. Mandrake or fedora (redhat) are excellent for this, as they install using a graphical interface.
The major mistake i see ALL THE TIME is that people install *.nix based operating systems and expect them to be similar to windows. They are nothing like windows. GUI's attempt to reconstruct that familar environment, but theres alot of stuff to get use to. Learning command line is a must if you want to become a pro, and get use to using the terminal to launch programs, su into root, etc. Fonts will take a while to get use to, for one, and it took me a long time to be able to browse forums with out cringing
. Another thing, you can't just plug in peripheral devices, soundcards etc and expect them to work. Everything needs to be configured. It doesn't take long, but in the beginning it can be a pain in the arse.
I'm rambeling, my apologies.
Good luck, and if you have questions don't hesitate to drop me a pm.