1)Two kinds of memory - "random access memory", and "secondary storage" (your disk).
The processor chip - the CPU - can only work on data and instructions that have been loaded into RAM. The CPU cannot execute a program which is "on the disc", and has not been loaded in RAM.
If you dont have a lot of ram, some programs may not fit into the available ram space. In this case, the computer will bring in a portion of the program, and run the program until it needs data or instructions in a part of the program still on the disc. The computer will then bring in the needed portion of the large program, re-using the ram that had been in use for the first part of the program. Broadly speaking, this is sometimes referred to as "paging" or "swappin".
Its a slow operation. So the more Ram you have, of almost any speed, the faster your programs may run, since swapping will not be necessary.
2) RAM has a "speed" associated with it, and the CPU and its integral bus have an associated "speed." With one exception, there is little value in having a RAM speed in excess of the bus speed. Because there is no reason to be able to deliver twice as many bytes in a millesecond than the CPU can handle in a millisecond.
3) The exception is "overclocking." The CPU has a manufacturer's speed rating. This rating is the lowest guaranteed speed at which the CPU will run. But, many CPUs off the assembly line will run much faster. To run a cpu faster, perhaps 30-50%, you need to install a faster clock crystal ....... and faster RAM.