After testing my current machine, which Vista upgrade said was perfect, I bought and downloaded Vista Home Basic. It wouldn't work. So I just decided to stay with XP for the next few years. Got my money back, no problem and that is that. Just my experience with Vista upgrades. I don't know much about OEM installs.
For myself, I bought a copy of XP Home last month so that when I get a new one this summer, I have it available to load as I expect copies of XP to be as scarce as hen's teeth. I know MS is going to support XP for years to come, but getting a new computer with XP is difficult now as the dealers sell only Vista. So an answer to this question will be useful for me then, too.