The Truth, since you never purchased a valid copy of windows, you can't legally use an "upgrade version" according to MS EULA. You'll need to purchase a full copy of windows 7.
Now, if your computer had a valid copy of windows installed before you installed the beta, you can use an "upgrade version" to installed windows 7 over an "in place" beta/RC install. What you would need to do, is called "the custom install option." You won't lose your documents, but will need to copy and paste them from the windows.old folder on C: drive to your new document, music, videos ect. folders of your new windows 7 install. Your programs will need to be re-installed completely.
It's a myth that a valid copy of xp or vista must be installed at the time of the "windows 7 upgrade," to comply with MS EULA. You must own a valid previous copy windows for that computer, it doesn't need to be actually installed at the time of the upgrade to windows 7.
This if from a MS big wig:
http://blogs.msdn.com/mssmallbiz/archive/2009/10/27/regardless-of-what-any-hack-says-a-windows-7-upgrade-is-an-upgrade-what-you-need-to-know.aspx?CommentPosted=true#commentmessage