Have a read of this thread as an example click here you can put XP on it but only if the drivers exist for XP. If they don't you'll just screw things up.
As to whether you can add memory it depends whether theres a slot free. If a laptop has 2 x 512mb giving you the 1 gig then you would have to remove one or both to upgrade it. If its a single chip then there should be one space free. In short it depends on the laptop and its worth checking before purchase.
If you have a dual core processor and 2 gig of memory you might be happy enough with Vista. If you really want XP then try to get one that comes with XP. I know Novatech do it and I know Dell do it and I'm sure theres others, what the warranty is though I don't know.