The best deal out there now is Windows 7 Home Premium Upgrade version Family 3 pack for $119 on Amazon. That's three licenses that can migrate for life for less than the price of one.
If you buy Windows 7 and it's the same version, just insert the Product Key at Computer>Properties activation link.
If it refuses the key, do the quick registry edit given here to activate Upgrade version which has been installed to a wiped or clean HD: Clean Install with a Upgrade Windows 7 Version
If you have a different version which is more expensive to buy a key, you can clean reinstall with your new Home Premium DVD folllowing these steps (which apply) to get a perfect install: Reinstalling Windows 7
There is also a workaround to change versions of Windows 7 to your licensed version if you need it.