Microsoft's gift to dual booters is that Vista, the poster child for
operating system instability, deletes its system restore files when you boot
into XP.
That was a deliberate and totally unnecessary move on the part of Microsoft.
That alone qualifies Vista as deliberate damaged goods.
What you will see in XP is time by your stop watch faster operations on
anything that involves accessing the hard drive, among other things.
On the other hand, what you will be missing in Vista is not a whole hell of
a lot.
And just remember that Windows 7, Vista in sheep's clothing, is just around
the corner.
Unless you need Vista, and the only "need" I can think of is to see the
underwhelming gee whiz GUI features in Adobe CS4, you will have less
problems overall if you make XP your main OS and only use Vista when you
want to be frustrated, as in networking, file copying and anything else you
need to do with your computer. On machines where I have a choice I always
wonder why I boot into Vista: too often I am forced into XP to get actual
work done.
You need a third part partitioning program, of which there are many, but
make sure you back up your Vista installation, or at least your necessary
files, in case something goes wrong.