help on bootcamp and parallels on leopard

I would appreciate help on basic questions about installing Bootcamp with Parallels. I presently use Windows XP on a Macbook with Tiger 10.4.11 and Parallels 3.0 (runs fine for simple apps).

I want to install Leopard and to have the choice of using Bootcamp or Parallels. I understand I need to create a partition for Bootcamp. Do I need to first install Leopard and then create the partition?

Will this erase the content of my present hard drive?

Do I need to de-install Windows and Parallels before installing Leopard and creating the Bootcamp partition?

If so, any warnings/suggestions about this operation?

What partition size is advisable if I later want to upgrade to Vista?

I understand Parallels will be able to access Windows in the Bootcamp partition. (No need for installing Windows twice.) Right?

After installing Bootcamp and re-installing Windows, will I be able to download the latest version of Parallels or should I first re-install the CD version of Parallels I purchased in April 07 and later upgraded to 3.0 online?

Many thanks to the experts who care to help on some of the above!