EDIT: Stay away from any upgrade edition as it requires XP to be installed, my experience upgrading over XP (shame on Microsoft for forcing this on people) has been it stinks, don't do it.
There seem to be loopholes for that.

Btw, I'd recommend everyone to do a fresh install of Vista if possible. For one reason to avoid clutter from previous Windows versions, but also because the upgrade is slooooooooow. But a fresh install is done in 20-30 mins.
The only install path for the Upgrade versions of Vista is to install a fresh copy of XP first, or install over a currently configured system otherwise there is no way to install an upgrade version of Vista by formatting and starting fresh, if you've found one post it.
Install without previous version is possable with upgrade version it was well reported you should be able to google for it failing that cnet,techrepublic,pcmechanic,pc world etc
It works by installing as a trial this is done by not putting the key in which gives you a limited time,then on restart install as full version and include the key & viola vista installed without previous windows or something like that.
OK back on topic unfortunately my machine can't run vista which pisses me off immensley
& i am in no doubt it is far better than xp after all it is a later OS it's only common sense so i can't wait too upgrade this machine or build a new one (send donations too *scorp needs a new comp fund*

), from what i've seen on the shelves it aint look'n too shabby tho i gotta say i like the thing ms showed off a couple weeks back where windows were moved by hand