Why not make it 1/2 HP across the board. PCs 1/2 hp and monsters 1/2 hp. This would be so easy to implement and it seams fair.
Just as a note this method greatly increasing the power of focused firing. A group of npc archers could take down a party member in one round, and vice versa on the pc end.
Not saying that's a bad thing mind you, just keep in mind that you can't just cut both sides equally and expect the gameplay to be the same but faster...it won't be.