The question you need to ask, is it worth upgrading at all.
If you have an XP computer that is working fine then you will gain almost nothing by upgrading, and may give youself LOTS of problems.
I work with computers all day, and did my first XP to Vista upgrade last week on a PC just over a year old.
Vista could not find the sound drivers, so now I have a working Vista with no sound.
My advice to anyone would be:
If you are buying a new computer that has Vista on it, fine.
If you are thinking of upgrading to Vista DONT.