Dual Booting Vista

I have a question. Recently my mother bought a computer that had Vista pre-installed. Now I plan to upgrade the crappy 3D card thats on it I will probably send it back to HP-Compaq because it's still under warranty. However I want to dual boot Vista with XP because Vista has atrocious compatibility with pre Vista applications and I'll be damned and in my grave before being limited to ONLY Vista applications. Vista's compatibility mode is terrible it might as well not exist for me. In anycase I wish to Dual Boot but I remember from earlier college classes that you need to be careful the order you install the Operating Systems in because it rewrites the registry and MBR everytime a OS is installed. Now since Vista is already installed on the system I can't dictate what order the OSs are installed. I was wondering what is needed to properly work around this situation and what I need to do in order that both OSs work together with no problems. Note The Vista partition came formatted with NTFS and when I install XP I plan to format it with NTFS as well. Vista is on the main partition of C:, there is a Recovery partition on D: and there is also a DVD RW already assigned the E: designation. I planned on getting a external USB hard drive to avoid voiding the warranty my mother has on the computer.