I have two computers running XP without any problems, and I have a laptop running Vista without any problems.
If you buy a modern computer today you shouldn't have any problems with Vista. Most of the people that are having problems with Vista are using older, less powerful computers that can't handle the more advanced operating system.
Vista is more secure, has a few neat bells and whistles, and can handle some of the latest hardware components that XP can't. Other than those things there really is very little difference between Vista and XP from the user's perspective. There is no need to buy an upgrade from XP to Vista. Just stick with XP until you get a new computer.
When you are ready for a new computer, the best thing to do is buy the computer components and assemble them yourself. You will get a much better computer for a much lower price. It is very easy to do because even I have done it many times myself.