I was never a fan of called shots because if they are allowed, everyone will go for called shots to the head. (Or in whatever system used, for the one that prevents the foe from fighting back the best)
The simplest solution that I found to reduced headaches of adjucating unfair called shots is simply not to allow them; Going along with the "I can swing my sword far more than 5 times in 6 secounds" train of though, I find it best not to have the combat rolls not be a literal translation of what is happening, but the sum of it.
Example: A fighter with 4 attacks isn't attacking four times each round, but rather is engaged in a sword fight with his foe, and is parrying and dodging, and those four attacks he is entitled to a round are simply the number of attacks he can make that have a chance of landing, the sum of his skill, feinting, and reflexes.
I doubt a trained 20th level fighter's entire plan of attack is to nebulously swing his sword in the direction of his foe, hoping to defeat his foe's defences, and pierce his armor, while standing still for 6 secounds.
Called shots could simply be the critical hits, or the high-damage power attacks. A critical hit/wound table could best be used under this system to generate really gritty combat.
I found an extremely extensive critical wound table a while ago on a site, I think it was for hackmaster, but I converted it to d20. I ran it in a game, and it made for some brutal fights. (although the table was a bit unbalanced, either it wasn't worth rolling half the time, or the other half it was a total character crippler) The players though the table was pretty cool.
Sadly, I don't remember where I found the table, but if you wanted to use called shot rules, perhaps something like that could work.
Honestly I think for called shot rules Power Attack would be the best idea. It's nebulous, fair, and already balanced withing the current rules.
Called shot rules generally nudge the game a bit towards realism, and realism, from a balance standpoint is unfair. Realisticly, A direct Great axe hit to the torso kills, period. So the more "real" you make it, the more deadly combat is going to get. Just remember that when you plan out the evil archvillain, and flesh out every aspect of him, only for the PC archer to called shot his head and kill him before he can finish monologging.
