This is kind of a transition time for Windows. Win 7 will be released in Sept. It appears to be everything Vista was not -- including being stable. Many laptops running Win XP are expected to have hardware and possible driver problems making the change. XP cannot be upgraded to Win 7, it requires a total disk wipe and build up, all data and programs will be lost. Laptops running Vista will be a very simple upgrade, no lost data or lost programs.
Apple is also planning to unleash their Snow Kitty OS in Sept (my, what a coincidence
). I have no idea how that will effect existing Apple laptops, but given Apple's history, probably won't be much of an issue at all, wouldn't expect any major problems like there will be with XP going to Win 7.
Due to govt contracts, MS is obligated to support XP for a few more years. The problem is new hardware & drivers between now and discontinuing support for XP may or may not be compatible with XP.
Sooooooo ... I would be looking at either Apple or a MS Vista OS on whatever laptop I got. I would stay away from Win XP machines, now.