Yes you can upgrade install from the RC with the RTM (assuming your RTM is also Ultimate) and then activate the RTM by inputting a valid retail key (full or upgrade) for your version of the RTM. If your version of the RTM is not ultimate, you can still upgrade but I believe it must be done as clean install.
To do it as an upgrade instead of a clean install, there is a little
workaround (published by MS) required.
If you don't have a lot of apps installed on the RC version, it probably would be better to do a clean install, but if that would be very time consuming, you may want to give an upgrade install a try and just see how it works out. It seems to have worked out pretty well in my case. Anyway, there is nothing to stop you later on from doing a clean install if it doesn't work out to your satisfaction so you really don't have much to lose by giving an upgrade install a try.
Of course, whichever route you take, it's always a good idea to have a complete back up on external media of all important stuff you don't want to lose, just in case something goes terribly wrong with in the install process.