If you had the choice between XP and vista? XP all the way. Vista is too new, too hungry for today's average PC. All that grunt wasted just to run the OS. Some people here are running Vista without issue, but i would at least wait for another year or so yet. While MS are supporting XP, stick with it if you can.
Personally i'd buy a generic PC - eg all made up of standardised parts, not a pre-built HP or other brand-named PC (dell are an exception i think). This way there is no proprietary junk or bits to limit future upgrade options.
Also, personally i'd wait for DDR3 to arrive cheaply and go for a DDR3 Mainboard, and most likely an Intel Core 2 Duo as they are much faster than equivilant AMDs at the moment - however this may well change in the future.
Buy a PC with a mainboard which gives you a good upgrade path too.
When the time comes, go visit your local PC shop and get one built for you - or build it yourself

Things change rapidly, so do your planning as close to the purchase time as possible, and make sure you have solid upgrade options.