With modern bus speeds of 200mhz quad pumped and hypertransport buses though, the difference becomes pretty small, and in fact if you have to start lowering the timings on memory, PCI buses, etc. to make the system stable, your overclock by raising the bus speed may end up with less performance had you left everything alone and just raised the CPU muliplier. (if your CPU is unlocked, if it is not, you have no choice)
Is there a difference between 133 and 200 by virtue of bus vs mulitiplier. Maybe, maybe not! Only trial and error with each system and benchmarking can really reveal that! Depends on the rest of your hardware, and how well it reacts to the increased system bus!
From say 200 to 220 should you go by bus or by multiplier? See this is where it gets complicated. Raising bus speeds more at this point does not impart much speed increase at all to anything system wide, except the processor, and it might do more harm than good by causing instability with other components. It may be better to raise a multiplier on the CPU and leave the bus speed alone when you reach these bus speeds.
This is really sort of a left over trick from days gone by, when bus speeds were a lot lower. By raising the system bus from 33 to 66, or 100, or 133 etc. back then did impart huge, huge gains in overall system speed. Much more than just raising the multiplier on the CPU to achieve the same CPU clock speed.