If you buy a reasonable laptop with a good chunk of memory and a dual core processor then it will run Vista fine.
But I've seen 2 cheap laptops, about the £299 mark, with poor Celeron processors and little memory and they crawled. Even adding another 512mb to one didn't make that much difference. It did speed up but not as much as I hoped and personally it was far too slow for me. And they both had Vista Basic too.
Be all means buy a Vista laptop but do buy a reasonable spec one, I've seen some not bad ones for maybe £400-500.
My advice. Go to the likes of PC World, see some demos on different laptops including how long it takes to startup fully. One of my friends that has the cheap laptop and that I added more RAM to takes a good couple of minutes to fully boot up and before the RAM upgrade it was more like 4 minutes and thats terribly long when you want to just use the damn thing. Once you've done that make your own mind up.
Dell I think still do XP laptops but you'd have to check their website to be sure as Microsoft was stopping it on new machines and I cannot recall if its come into affect yet. Novatech still sell them though click here and I like Novatech a great deal but have only ever bought PCs off them and never a laptop.