I'm going to skip over my initial response to this, because I realized I misunderstood what you were asking. From the checking up I've done at Dell's site, it looks like they don't offer any sort of Vista on disc option anymore, I mean it seems like they sell their systems with Vista pre-installed on the computer with no backup, no way to restore it from disc. When I got my laptop that was an option that I believe I picked, but now it looks like it's no option at all.
So.. it would seem that if you wiped your hard drive clean to install XP on it you wouldn't have any way to reinstall Vista at a later date. This is only a guess based on some preliminary checking at Dell's site. Gateway's site seems to mirror this.. and actually, I do seem to remember reading that MS wasn't going to be allowing manufacturers to include restore discs with Vista systems. I do not approve of that in the slightest.. a hallmark is Windows systems is that they run best if you wipe them clean and install them fresh every so often (I don't actually do that though, I do an insane amount of customization to my systems, it would take me days if not weeks to restore everything I had done before). What MS is doing is trying to force people to purchase computers with OEM installed Vista on them, and then at a later date if they want to reinstall a fresh copy of Vista make them purchase a retail boxed version as well...
A similar trick seems to have been used on the upgrade versions of Vista. From the fine print it would seem that you can only install that once as well, unlike all their previous upgrade versions.
Microsoft don't want people to be able to put Vista away for a while, they want to force it on the world NOW. And they want to prevent people from going back to XP. Another aspect of the Vista upgrade is that the moment you install it, it invalidates your XP license so you can't go back to it (which is also why I suspect you can't reinstall it, you have to have XP already installed to use the upgrade, but you shouldn't be able to reinstall XP because it kills the license).
As a side note, Dell offers SOME systems with XP as an optional OS. Mostly it seems that their business systems are the ones that can come with XP. Since you already own a copy of XP that isn't a great improvement, but.. also Gateway seems to have it as an option, although you can't do it from their website, they say you have to call them to order XP systems.
If you did purchase a Vista system and wiped the drive clean (accepting the loss of Vista), you would at least end up with a much faster, cleaner running system since it wouldn't be loaded down with all the trialware that manufacturers load systems down with. You'd have to find any drivers that might be needed, like for the sound card or the video card, and I'm always uncertain about how easy it would be to find them. In the case of my Dell laptop it's easy, their website makes them all available after I tell them what kind of laptop I have.
Personally.. my plan is to resist Vista as long as possible. I'd say I'll NEVER touch it, not even with a twenty foot pole.. but with the way MS operates, and the way the justice department has totally fallen asleep at the wheel of the whole issue of protecting the public from anti-competitive behavior, I may have no choice at some point. Or rather my only choice may end up being Mac or Windows, I suspect MS is working to lock the PC architecture down so that other OSes like Linux (or even XP) can't be used on it.
It may come down to grabbing the best of the last of the open architecture hardware and building myself the best last generation system I can, and seeing how long it lasts me.. hoping, in the meanwhile, that if nothing else the international community, like the EU, might be able to force MS to change their ways.