A standerd setup with good pads and qualty disc work well. Depends on budget and engine spec. 4 pots are better but are more money. Also change to braded hoses at the same time, much better pedal feel.
Depends on what your're doing with the car but standard S brakes are good enough for anything, even circuit racers, with or without a servo. You do not need fancy calipers or discs.
Would have to say 4 or even 6 pot bracks do give more braking. On the track I ran both and could did get an advantige with 4 pots. But did not realy notice any advatige on the road.