If your buss is only capable of 100MHZ then the information can only be passed at a speed of 100 million cycles per second. Not much because, what the average person does with a computer and the programs that are available for the computer, hardly load the processor so the processor is able to accomplish what is requested of it very fast. However if you are a rocket scientist and your computer is doing trillions of precise calculations or if you are or the power company sending out monthly bills the CPU speeds might be something that is very important to you. The cache that I previously discussed is called disk cache and we learned to clear it out. Processor cache is memory that is added to the CPU and it's very expensive, but it is the fastest memory around.
Modems can be external or internal.
External modems have cases around them. A modem in most instances connects you to the web. Of course if your computer can't handle a 56k modem then it may be time for you to get a new computer.