Dlowan, I'd wait out the Vista operating system. At least long enough to see what the new system is all about.
A question I hope the likes of Nick and Robert might weigh in on.
The operating system thing really depends on your situation, in my opinion. If you buy a name brand computer like Dell, it will come with Vista, and there is no reason to downgrade.
However, you mentioned you might have someone build the computer for you, correct? By that I assume they will just buy individual components, rather then a name brand like Dell. If you go this route, then you can pick whatever operating system you want. Do you already have a license of Windows XP? If you do, it might be worthwhile to just install your copy on the new computer. It will save you the money of buying Vista, and it is a very capable OS that you are already used to.
The new version of windows, 'Windows 7' is in a pre-beta state, and is likely to be released early 2010.
I personally prefer upgrading components over buying a whole new system, but you need someone knowledgeable to do that. The advantage, is it saves you from re-purchasing perfectly fine hardware. For example, I built my computer 8 years ago, and still use it. There isn't a single component that is original anymore, but I've never had a major expense all at once. One year I might upgrade the motherboard and CPU, the next I might add some RAM, I replaced the video card about 6 months ago when it failed, etc.