Setting the North Bridge higher will yield performance gains. If you just up your CPU multiplier to 19x (3.8Ghz) and leave the NB alone, you will see a performance gain. However, if you up the NB frequency, you will see more data throughput, and thus, performance will actually be better than if you just increase the multiplier. Remember, though, that if you increase the NB frequency, you will have to increase the NB voltage as well. Since the NB is on chip on AMD processors, it will increase heat output on your processor, so be mindful of that.
It's a balancing act between heat and performance, though the sweet spot for the NB is around 2400-2600mhz. Above that is nice if your board/chip can handle it, but remember that heat will increase dramatically if you set the NB voltage over about 1.2v.
That's been my experience. Check out AMD's forums if you're dead set on becoming an overclocking pro on these chips. Those guys have a lot of insight.
For reference, I've got my 965BE at 3.8Ghz. I left the multiplier at stock (17x) and increase the FSB to 224mhz. I've got my FSB multiplier @ 11x, so NB is 2464mhz. Memory is at about 1500mhz.
Voltage at this level will vary based on your chip's capabilities.