i would go for a very limited use of cameras. rugby league/cricket employ video refs, but only use the evidence in certain circumstances, leaving the rest up to the ref/umpire.
perhaps 'over the line or not' decisions and offsides only should use it. wait until the ball goes out of play, then refer it to the 4th official. got to say that in relation to handballs, fouls etc it is pointless in my opinion, as most end up being a matter of opinion anyway. there are some like sam who will complain about the (in my view also) blatant handball by ehiogu, but im sure some boro fans will continue to claim it was ball-to-hand. clarifying the rules is the way to tackel those issues.