Northwood is a name for a CPU, there is no Southwood. So I'll assume you mean Northbridge and Southbridge.
The Northbridge contains the memory controller, AGP controller, and an a link to the southbridge. The southbridge contains the IDE controllers and PCI bus, allong with controllers for your ports (COM, USB, etc.).
The Front Side Bus used to refer to the "bus" between the CPU and RAM. But that's really 2 busses, the one from the CPU to the memory controller, and the one from the memory controller to the RAM. So now it refers to the bus from the CPU to the memory controller of the Northbridge.
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