Most new computers that come stock with Vista have the ram available to run it(mine had 2 gigs). Do you honestly think Dell would install Vista on their PCs if they thought it would make them look bad? If someone's computer is running slow because of Vista, they won't be blaming Vista, they'll be blaming Dell.
As far as old computers are concerned, the person who actually bothers to upgrade the OS on their old PC, will probably have a top of the line PC that actually has the RAM to run it.
The problem with this is that I want my RAM to run applications on it, not just the OS alone. I want to say to Vista: "fuck off, this is my RAM, for my applications, to solve MY problem, not yours".
We don't buy RAM to run operating systems, we buy RAM to run software, and OS must be our "Santa's little helper", not "Santa's rape-me-in-the-ass hairy inmate".
As far as old computers are concerned, the person who actually bothers to upgrade the OS on their old PC, will probably have a top of the line PC that actually has the RAM to run it.