Hello Neoglyphix, and welcome to Seven Forums.
Sorry, but no you cannot use a beta key on a RTM (retail) version of Windows 7.
The RTM is available at your local retail store. If you have a friend that has a copy of Windows 7, then you can use their Windows 7 installation disc to only install and not activate on your computer for a up to
120 days trial period.
Hope this helps,
Shawn