You can still buy XP loaded machines but you have to hunt for them. Vista hasn't been 100% successful and the trade aren't happy about that, so they supply 'XP downgrades' in order to stay in business and keep the money coming in. Just search 'xp laptop' or 'xp downgrade' and you'll see. I just bought one.
The guys here tell me (if I understood right) that you can load a standard OS to a 64-bit machine and it'll run fine. In other words you could load your XP 32-bit OS onto the 64-bit laptop. Don't know if you knew that. Yes there might be a legal issue there but morally and ethically you are 100% right to do so, so I certainly don't have a problem advising that in public.
Re. wiping your laptop by overwriting it with an image, that's a bad idea. There must be better options than this. Yes you could mess around doing partitions etc but to me that's (a) not a solution as restoring a partition to a bare metal disk is not guaranteed to work, and (b) it misses the whole point which is that you should have a 1-click image create and a 1-click whole-disk restore - well, ideally of course
There are various reasons why this is not a good idea and I can't agree with anyone advising it (which I've seen). Just because someone makes the mistake of advising it online doesn't make it right. Sure, it's OK if you are talking about a 'disposable' install, in other words a brand new one also with the original installer disc right there. I've been writing about disk imaging online since 2002 but I never advised anyone yet to wipe their main disk with an image
In your position I know exactly what I'd do - load XP standard to the laptop. For me that would be the perfect solution. If you want to run your 64-bit Vista and test an image made from it, you need a spare disk on a 64-bit machine to test it. That's the result of being out on the leading edge.
From an engineer's point of view that's the last place to be - but I guess there are a lot of market forces out there...