There's a couple factors involved...
Firstly, how much speed can the arm build up in the swing? Grevious-esque prosthetics would be at least human speed, and possibly a bit more (depending on how much he was using the Force), while Darth Vader-esque ones would probably be slower. We never really see Darth Vader move quickly... Initial speed translates into kinetic energy on the impact. Obviously, the harder the hit, the more impact damage (breaking bones, etc) is going to happen. Based on the movies (the only SW canon I'm familiar with), I'd guess it to be similar to a conventional human.
Second is follow-through. A large portion of the initial impact's energy dissipates as damage. Actually throwing people around happens in the follow-through, which is effectively a very short, fast shove. There's two things that affect the amount of energy that can be transfered - the weight of the cybernetic arm (a heavier arm will have more momentum, and deliver more shove at the cost of hitting power), and the strength and speed of the actuators (which can deliver something of a push during the contact time). Once again, guestimating from Episode 2 (where we see Grevious unable to overpower Obi-Wan), I would guess that Star Wars prosthetics aren't that much more powerful than regular humanity.
Finally, fist shape and strength. A blade will penetrate a skull deeply, while a brick-shaped fist with the same force could just crack it. Durability under impact loading is also important - if the fist deforms, it can enlarge the contact area, lessening the point load.
Fortunately for us, the human skull is very well designed. The average human, or even the average martial artist, really doesn't stand much of a chance to break it. Most martial art strikes to the head aim for softer targets - the nose, which is cartilage, and can be broken from the skull. Eyes. The temple is an interesting target because it's where a fairly major blood vessel enters the brain - a hard stroke to the temple can disrupt blood flow, without much risk of penetrating the skull itself. By far the easiest way to penetrate the skull is with a long, sharp object, delivering the maximum amount of impact energy to the smallest point possible. That much can be managed by a human with a good swing and a stiletto. A brute-force punch... For the vast majority of martial artists, there's no way a flat-fisted punch could really break a skull. Fractures would be the worst assumable injury.
As discussed above, I estimate Star Wars prosthetics to be similar to human strength and weight - the only real hope for a skull-breaking punch would be with a very non-human fist shape, or a swiss-army-knife stiletto attachment. Obviously, as they're mechanical things, one could build a massively powerful prosthetic arm, but see hS's comments concerning balance and leverage. There's also a weight problem - having a two hundred pound arm can be a real drag. (pun intended)