Hi Dan--
Less problems if you install the older OS first. So you've done well there.
If you make a new partition as said, you will have no problems doing a dual
boot and will get a choice of which OS you want to boot to. If you can, run
the Vista DVD setup from your XP desktop, and you will get the option to
select the new partition/drive you've made for to install Vista to. That
way your drive letters won't change when you're on your Vista boot. If you
can't for some reason, run setup from restart (this is necessary on some
older computers but it installs just fine).
I find this tip helpful:
You can shortcut to your XP boot from the vista desktop and drag the icon
for shortcutting from the folder address bar:
XP Drive\Documents and Settings\Dan's XP Profile\Desktop
When on XP you can shortcut to the Vista desktop via:
Vista Drive\Users\Dan's Vista Profile\Desktop
Good luck,
CH