Steam on Windows 7
There are three main ways to get your games from your current Windows version into Win 7.
1. Format your drive, install 7, download Steam and ALL of your games one again and all your games will be there on your PC, this is clearly the most bandwidth intensive option.
2. Move all your games to an external hard drive and point to this location in Steam, install Windows 7, and Steam, then point Steam on Windows 7 to the same location. Done. No downloading of games.
3. Use the backup to disk feature then install 7 and restore from the backup. No downloading of games.
The 3 above ways are the quickest ways of upgrading, of course do backup your OS before upgrading and if you manage to do an in-place upgrade, e.g. Vista Ultimate --> 7 Ultimate, then none of this will need to be done. Except of course the initial backup.