If I get upgrade fever and decide to replace bits piecemeal (hey this buying bike bits is getting addictive ) I'm probably going to have to go from 7sp to 9sp. Forgive my ignorance, but does this generally mean a similar bottom and top with closer gearing in between or gearing the same difference apart but with a higher top and lower bottom?

I'm currently running SRAM ESP 3.0 shifters, and rear derailleur (I'm quite happy with these) and a Shimano front mech which seems to be the weakest part in terms of performance - I get next to no mis-shifts on the rear but the front isn't anywhere near as reliable (is this just the nature of the front?).

Richard