I agree with Dexter about Vista 64. I've had it on my home PC and it seems to be stable and I haven't had any problems with it. As long as it's an option, go with 64 and not the 32bit version. You can add more RAM if you wish. With the latest Service Pack that came out, I noticed that about 20GB of space freed up on my hard drive. Other users have noticed space freeing as well.
My laptop is running windows 7 and while it seems to be stable, it is hard to find compatable programs. You can use the Compatability Wizard, or Compatability Mode, and you may be able to get them to work, but in my opinion, it's not worth it. If you could buy Vista x64, and somehow get a free upgrade when 7 is released, that may be a good thing, but I believe it's only available when you buy a new PC.
Windows XP is a great OS. But I don't see Microsoft providing support for it too long after 7 is released. You can try to take a chance, and I guess it all comes down to what you want to do with your Laptop.
My decision would be Vista x64. But remember that 7 is coming out soon. If you wish to try it out, you may be able to still get a beta key and try it out for free. Then you can make your decision.