As mentioned above, Athlon 64s have the memory controller built into the core. The link between the CPU and other components such as the memory is called the Hypertransport bus and is calculated by multiplying the FSB speed by the LDT ratio:
200MHz x 5 = 1000MHz
The Northbridge is a chip which communicates between high bandwidth components such as the RAM and the AGP/PCI-E slots, while the Southbridge comminicates between devices such as USB, PCI, SATA, IDE etc. The two together are what is called a chipset.
That's how a typical chipset looks.