To boot correctly, Microsoft OSs have to be installed on the computer in the order they were released by MS.
If you install another HD with Vista on it, the boot loader on the original disk will not see the Vista install and will not give you the option to boot Vista. You could add the disk with Vista on it and then change the boot order in the BIOS from HD to HD every time you want to boot the other OS, but that gets to be a PITA real soon.
Just put the new disk in and install Vesta on it. It will see the original XP install and automatically configure it's (Vista's) boot loader to give you the option of booting XP or Vista when the computer starts up. I assume this is a Vista full install, not upgrade disk. If it's an upgrade disk is will "upgrade" your XP install to Vista and your XP will cease to exist. There are ways around this though.
It's very advisable to do what you're doing, keeping XP until you are familiar with Vista's quirks. It's a good idea to get all the Vista drivers (chipset, video, printer, etc.) ready to go before you install Vista too.