If you install and activate the RTM version, you will avoid the problems that users of the RC version will start to experience next March.
To legally activate the RTM version, you need to decide which version you want and purchase the software, which basically means purchasing a key for it.
If your RC version is activated, you could buy a cheaper "upgrade" version of the RTM and install it over the RC. When doing this, you get a choice of either doing an upgrade install (which would keep your programs, settings and data) or a clean install which would wipe everything and require you to reinstall your apps, settings, and data. A clean install is usually recommended although some, including myself, have had good luck with upgrade installing from the RC.
An upgrade install from the RC to the RTM is not officially supported but MS has provided instructions
on how to do it, however, since the RC is the Ultimate version, I think you would have to install the Ultimate RTM version to make that work.