Focused Attack Rolls

I'm having trouble determining how a character could strike (or attempt to) an opponent's hand or the ring on his right index finger (maybe not that specific.) This same combat subsystem must be applicable to other living species (same for humanoids as insects) and to non-living constructs (like doors and weapons?)
The most promising system thus far, has set percentages of accuracy:

The head is 40% harder to hit than the torso.

This allows more skilled characters to make more skilled attacks. Not just for the more critical damage(my mind just went off on a tangent) but also for tactical purposes:

The enemy wields a sword of instant death in his right hand and wears a crown of mind control upon his head. Character decides the sword is useless if enemy has no head and just goes for a chop of the neck (Difficulty +50%).

I would just like feedback as to the pros and cons of such a system.

Tangent that occured while typing:

What if you could make an attack roll against someones arm in order to disable it (just shock), or in order to sever a vital artery/vein (going for the kill) This would complicate the system but make it more lifelike. Each grouping, or limb, would have its own set of criticals based upon attack:

Arm (+ 25% to hit) for 0-20% over a hit, normal damage, 21%-30% over, 1 Critical...This would change my current system somewhat and be damn complicated. Maybe this is more applicable in a computer FRPG setting (cause I'm not doing all that)