The front side bus is a pathway that connects the processor to the northbridge chip in traditional processors. Everything is a multiple (or fraction) of this bus, so changing its speed changes the speed of many other things, including the processor.
In the case of the Athlon 64/FX, a new concept called the Hypertransport bus arises. This is the name of the bus that connects the processor to the northbridge, and once again, changing its speed changes the speed of everything else.
Your bios may call it the FSB (Front Side Bus) or the Hypertransport bus. (The value should say 150 or 600 I think) In either case, raise the value a little bit and the processor speed will also go up alongside with it.