On the older systems prior to DDR2 you would match the memory to the model cpu to see the highest fsb setting. That would be 200mhz for DDR400 PC3200 memory with an Atholon XP3200 Socket A model cpu for one example. PC2700 memory used would have to see the 166mhz setting instead. From a 400mhz fsb you would then see that lowered to 333mhz.
On the newer boards for AMD as well as Intel HyperTransport finally replaced the front side bus once and for all. The following explains how that was done and why the actual speed of the memory installed now has less of a bearing. OCers on the other hnad will use the slower 667mha memory while stock systems would see the DDR2 800 when planned out.
Front-Side Bus Replacement
The primary use for HyperTransport is to replace the
front-side bus, which is currently different for every type of machine. For instance, a
Pentium cannot be plugged into a
PCI bus. In order to expand the system, the front-side bus must connect through adaptors for the various standard buses, like
AGP or PCI. These are typically included in the respective controller functions, namely the
northbridge and
southbridge.
In theory, a similar computer implemented with HyperTransport is faster and more flexible. A single PCI↔HyperTransport adaptor chip will work with any HyperTransport enabled microprocessor and allow the use of PCI cards with these processors. For example, the
NVIDIA nForce chipset uses HyperTransport to connect its north and south bridges.
http://en.wikipedia.org/wiki/HyperTransport