You might want to know that when you export your configuration with Reaper's built-in feature from the preferences menu, it won't back up these things:
-whatever is in the "UserPlugins" folder (FFMPEG, ReaPack, SWS, etc.)
-your registration info
-the index for ReaPack (which contains all the info about what you have installed from ReaPack, so it knows that it can update those files)
-in Linux, the customizations to the menu/fonts etc. that you make by altering the libSwell.colortheme file
This is why I now just back up the entire contents of the Reaper resource folder (as Solger said). I zip it all and save the zip file as a backup. Then when I re-install Reaper (on a new hard drive/OS): I start Reaper once, then I unzip and copy all those files into the Reaper resource folder of the new install (overwriting what's there).
You could choose to back up the ReaPack index separately from ReaPack (it has an export feature for this), re-add your registration info, and be ready to re-install SWS and FFMPEG (etc.), so it isn't necessary to back up that entire folder. But I find it's faster and more foolproof since it does everything at once.
Once I forgot to back up my ReaPack index, then I had to uninstall/reinstall every script and plugin which I'd previously installed in ReaPack so it would index them again (and therefore be able to keep track of them and be able to update them). ReaPack didn't "see" things I had installed using ReaPack on the previous computer, so I had to make it "remember" by re-installing everything. After that I decided to back up this other way instead.