CFM is cubic feet per minute... standard measurement for how good a fan is. 6.5 seems really small, but I guess it's just for the northbridge...
I don't imagine it really matters much... if your mobo didn't come with a northbridge cooler then it probably wont benefit much from one.
Also, you probably wont see a difference in temperatures. I don't think that there is a sensor on the northbridge on any mobo I've ever seen. It might help circulate the air a little bit more, but it won't be more than a couple of degrees.