Well, it depends on what you want to do. The Differences between the north and south bridge are:
North bridge generally controls/manages things like the cpu,memory,vga card. Things that need to go fast basically. The south bridge controlls things like sound cards, I/O cards, and harddrives. Those are the main differances.
As for wich combo is best, it really depends what you want your machine to do . I dont know much about the southbridge choices today, but for the northbridge a p35 is a nice solid chip. It overclockes very well also. But say you want to build a hometheater pc. The a g35 would be better. It all depends. What will you use your computer for?
Also keep in mind, Nvidea makes northbriges also, which are needed for sli.