How long have you been actually running it? Have you done a lot of 3d graphics activity on it?
Chances are, if it has been put through enough paces, e.g. lots of heavy use, or running 24/7, putting a fan on it now won't make much of a difference except maybe another month or two extra of running time, but you may still want to do it so you don't find yourself looking for a new video card sooner than it is needed.
But if it has only been lightly used, such as very light 3d gaming and non-24/7 run time, putting a fan on it now will probably keep your card happily churning out the frames for another few years, if you so elect to keep it for that long.