Hey Baz,
Sorry to have misunderstood
If you have a system with everything you need working under XP then the main question I'd be asking myself is: do I really need a new system?
Personally I'd always get the latest version of Windows if buying a new system.
You may have to wrestle with it for a few nights to get everything going and there may be some hardware driver you need that's not available under Vista but that is the price you pay for moving to a new system.
In 12 months the majority of people will be using Vista (it ships on practically all new systems) so I'd definately get Vista with the new laptop.
Have fun,
Doug