I have never heard of anyone loading Vista or W/7 on their existing
machine. It is generally a machine sold with that OS bundled in it. I
assume they would use an adequate machine.
*I* loaded Vista on two existing machines. I was an official beta tester
for MS in those days. I never had serious problems with Vista.
We know you are special and a glutton for punishment. I was talking
about regular people who actually do useful things with their
computer.
There may be people who are dumb enough to ignore the system
requirements and load a new OS on an old machine but it you really
want to go fast, load an old OS on a new machine.
Before MS approved me for the Vista beta, I filled out a questionnaire
to "qualify" my gear at the time. It qualified. The original Windoze
betas were for punishment gluttons, as they arrived on about 20 hard
case floppies. Every week.