Yesterday I did this same thing for my brother.
He didn't want to lose his Steam so we didn't reformat, instead we just installed the 7 upgrade and it made a folder 'windows.old' that had almost every file he had before. And success, Steam ran just fine and all his games were still there.
As for clean install, you could just run the upgrade and whenever it's done, delete the windows.old file and it would be almost the same thing.