You missed something.
The guy bought a machine on ebay.
Sure, one could take an older machine and add a lot of RAM and a high end
video card
and probably get Vista to run OK...
But considering the low price of new machines..it's not worth it.
(At least for most people)
I've got a lot of experience building machines.
FWIF: I'm a Microsoft authorized refurbisher. I've built literally hundereds
of machines...
more than likely the number is over 1000.
I've done a three month evaluation of Vista and stand by my statement
that it's really an OS for the newest H/W out there.
A dual core CPU, 2 gigs of RAM and a high end video card are really needed
to take full advantage of all of Vista's features.
Sure I can tweak it and get it to run on an older machine...
but for the OP and his ebay computer...
XP is the best choice I'm sure.
It's a big mistake to get caught in the upgrade game.
Better to save your money for the next tank of gas!