Incorrect, fr34ky, the 120 mm is indeed the whole fan, but since the saw is a circle, you will just cut out the part for the blades, not the entire fan. The mounts will still have something to mount to.
a dremel is something like an oversized and overpowered engraver. they run up to 37,000 RPM and have many attachments to perform tasks from sanding, grinding, drilling and cutting to polishing and routing.