If you buy the OEM version rather than the retail version it can be a lot cheaper.
This company is selling Vista Business Edition OEM for �90 and the home premium editon OEM for �70 and I am sure you can find other sites doing the same (or cheaper)
http://www.eclipsecomputers.com/products.aspx? cat=35&subcat=159&brand=57
Dont buy the Home Basic Edition as it does not come with the new fancy AERO interface.
Dont be too much of a hurry to install it. I did an upgrade from XP on a spare hard disk and found my sound did not work.
There are still Vista drivers missing so some hardware components will not work, some software does not work under Vista (Nero 6 for example).
Unless you plan to just "play" with it I would give it six months before moving to Vista, that way many of the driver problems will be sorted out, and Microsoft will have probably shipped service pack 1.