Ubuntu is a good distro. For any hardware you are thinking of getting, some Google searching will turn up compatibility issues. Its usually better to buy your own hardware, rather than get a prebuilt system. Some PC manufacturers use some screwy parts that have less compatibility. Sometimes you can be fine; just make sure to do some research.
Don't buy anything ATI; their *nix drivers blow.
You aren't going to be able to do quite everything on linux that you can on XP (probably mostly gaming). Dual booting always works though.
AT:
Yes, there are boutique vendors that will sell you a system with linux pre-installed, but it'll cost you. Interestingly, Dell is currently floating the idea of selling systems with Linux, though they haven't settled on a distro yet, to my knowledge.
Its not that hard to install linux these days, however. Provided you go with a distro geared towards ease of use rather than elitism, of course...