What you have said shows me that you have a solid knowledge of the problems facing prospective computer buyers.
The latest this year that you probably know is that Vista has done so poorly with its commercial product that Microsoft that it is bringing out a new "bug proof" upgrade of Vista, probably in the next year or so.
However, this new operating system will require, as it did with Vista a different type of architecture or in plain words it will not fit in our present computers without expensive changes in hardware, a different video card, improved Ram and a different mother board.
American businesses have balked against these changes and so Microsoft has ANNOUNCED that it will soon sell their new WINDOWS 7 separately as a stand alone product.
That means all of us XP owners will be able to UPGRADE our PC's with the installation of Windows 7. (My guess is that Windows 7 might cost us an extra $100 in the box) but I suggest you wait a few weeks when it comes out and see for yourself how much better it is that your present windows and decide if it is WORTH the upgrade.
This is tremendous news that I received just a few days ago and it has been corroberated by reliable sources.
For that reason I suggest you buy an XP machine with to-days available windows and then, when the new Win7 comes out BUY a copy and put it on your PC.
That is what I and most of our computer club members are going to do at the University to which I belong.
If you have difficulty buying an XP computer just go to a local dealer, one who has his own workshop or lab. Or, as many of us do, buy a COMMERCIAL XP machine, the kind that were made for businesses. For example an IBM XP machine that was made for business is available in our city for about $300.
Most big box stores will NOT have XP and will try to sell you Vista. They will tell you that support for XP will quit soon. Ignore that since XP has been and is a wonderful and reliable operating system and will be used for many years.
Get an XP with 2 gigs of Ram, a 3.00 or closest Ghz speed and a decent video card in case you might want to play a game or two. Shop around for the best deal.
Hope this helps. Good luck whichever you buy.