Well, on the Hammer- technology, the memory controller, origiinally located in the so called northbridge is integrated in the CPU- therefore it's much faster than any other home-pc- processor.
Since you don't need an external mc any more, there's actually no need for northbridge/ southbridge- solution, and Nvidia's doing a singlechip- design- which I think is worse than VIA's K8T800Pro.
Athlon 64 (U mean the fx, I guess) doesn't make the way to the memory over the chipset, it's directly linked- so you can hardly talk about "fsb". You could call it memoryspeed.
The link to external features (the chipset) is done by Hypertransport, a pretty fast link. Right now I think it can do 6,4GB/s- or was it 8GB? I don't know right now.
Well, your second question: yes, the PC2700 will cut the Power of the A64, but not as hard as it did during the NForce2- time. You'll just miss some bandwith.