Vista install will give you the option of retaining your existing XP or reformatting and doing a clean install (but only if it's a Vista full install disk). With an upgrade disk, you have no choice but to wipe the XP. (but, well, there are ways..........)
It's a good idea to dual boot XP and Vista until you are comfortable with Vista though.
To do this just install Vista, it will be aware of the XP install and will automatically give you a boot menu where you can choose XP or Vista on each boot up.