Hmm. Who says Vista needs a really powerful computer? I'm running it on a PC which doesn't meet Vista Capable requirements in memory and graphics (it's from 2001), and it runs just fine. Sure, I've had to disable the search index, and no Aero or DVD/Movie Maker for me, but everything else works as advertised.
I am really wondering why anyone would run a half-disabled OS on an old Machine instead of running XP (which has been stable as anything).

You could even play your videos on WMP with it!
As long as XP is still supported by Microsoft and still receives security-updates, I don't see the need to jump on the bandwagon. In fact, if Win95, and 98 were a bit (ok, lots) more stable, and still would receive security update, they would be perfectly sufficient for the computing needs of most of the people.
Somtimes this year I will most likely have to purchase a new computer as my laptop starts to fall appart at its seams. Vista will most likely come preinstalled with this computer and I will accept it the same way I accepted Win98 after Win95. But I will certainly not go out now, spend 400.- on an OS and force it on my old machine.
Yes, this thread has been resurrected, but it is probably more relevant now than it was last year.