I thought I had to enter "diff" in the diff tool field, but I was wrong: it didn't work. Apparently RapidSVN expects a graphical application instead of a text-mode application that runs on a terminal.
But fear not! There's an application for this. It's called meld.
sudo apt-get install meld
And in RapidSVN → Preferences → Programs → Diff Tool simply enter meld. Now you should be able to right click a file and select the Diff... flavour you prefer (to Base, to HEAD or to something else), and get the diff between the versions!
You might remember I always say I tend to use the command line for dealing with SVN, but sometimes I feel lazy (specially when dealing with many files at the same time), and RapidSVN is very handy to get a quick overview of what's changed. Besides, it doesn't break randomly like Subclipse/Subversive whenever Eclipse is updated, and can be used outside Eclipse, for non-Eclipse projects. Thanks to mrdoob for the cool app recommendation :)