You can just buy the upgrade version of W7 which keeps all the files, settings, profiles, etc, stored on the pc and upgrades everything around them.
Though a clean install of any operating system will always run a lot better than an upgrade but that depends on if you can be bothered backing everything up and re-installing everything from scratch!
Improving on my question I suppose what I really want to know is what is going to be better for video editing in a laptop, fast processor/dual core processor?????