I have been through the same process. Started with media center and firstly did an upgrade directly to Vista home premium. This failed the first time possibly because I was impatient half way through the install. It worked the second time, but Vista was not reliable, and so I went back, and did a clean install for Vista on a separate disc giving me a dual boot option. This works fine and has been reliable since March. System restore works on both Media Centre, and Vista, but each time I opt for Media Centre it wipes out all the system restore history on Vista.
So I have to remember to do a system restore as soon as I go back to Vista. But Vista is now my main OS anyway.
Hope this helps