[KarmaKiller]

No. Most all Power Supplies have the same mounting holes. So unless you get a insanely large unit, to where it won't actually fit in your case, you won't have to change cases. A 300w PSU is the same size as a 600w PSU. It's only when you get up into the 800w+ units that they start to become larger.(they are a bit longer, but not any wider)

As far as dual booting, that's pretty easy. Make sure you have 2 different partitions setup on your hard drive.(Or 2 different hard drives) Install XP first, on one partition. Then when you want vista, pop the disc in, boot from the CD, and select the other partition.(the one XP is NOT on). Let the install go through, and when you start up next time, it will ask you what OS you want to be in. It'll have a little menu come up upon start up.