I believe you are referring to IO operations for processing purposes, and I'll attempt to give a simplified layman answer.
Assume the processor is a meat-grinder in a factory, and assume RAM, hard disk are like the conveyor belt system feeding unprocessed meat to the grinder to be ground.
Assume the conveyor belt has two parts -> the slow-but-wide part, and the fast-but-narrow part. The former alludes to the hard disk big storage but slow speed, and the latter is referring to memory's small storage but high speed characteristics.
So...
HARD DISK CONVEYOR (WIDE BUT SLOW) -> RAM CONVEYOR (NARROW BUT FAST) -> GRINDER (PROCESSOR)
When your increase your RAM, it is like widening the RAM conveyor, thus the grinder can potentially receive much more at one go for processing.
If your RAM is low, it means that while the RAM conveyor is fast, it is extremely narrow, thus the volume of meat pouring into the grinder is little. At the same time, meat might potentially choke at the hard disk conveyor points (in short meat that is supposed to be on the RAM conveyor in a well-optimized system is actually still on the hard disk conveyor - a.k.a paging/swap file).
To sum an answer all up in a hopefully easy to understand sentence :
The relationship between RAM and processor and why programs run faster is simply because with more RAM, more data to be processed can get to the processor faster.
If the size of the system memory is equivalent to how wide the RAM conveyor is, then the Frontside Bus (FSB) is equivalent to how fast the RAM conveyor goes.
Whew! Hope this answers your question!
Best Answer
The CPU processes (performs instructions on things, such as adding) stuff in memory. RAM is just part of the memory pyramid (see below). So when you are processing lots of data, that data ( or maybe large portions of it) will likely get loaded into RAM so it is ready for the cpu, this is to speed things up because RAM is faster to access than storage devices. So CPU usage and RAM can often correlate, but don't have to.
A basic example might be an image editing program. I load up my 20MB jpeg, the program reads the entire image, and the OS keeps that in RAM for you (all working memory looks the same to the program, the OS decides if it goes to the page/swap file on disk or RAM). So the image is in RAM waiting to be processed, but I go for coffee before telling the program to apply some silly filter, so the CPU isn't doing anything: high RAM low CPU.
I come back, apply the filter to add some bubbles to the image, and the CPU goes to 100% and even more memory gets used because it keeps the preprocessed image in memory, so I can undo the change I just made. High RAM, high CPU.
When the program is done adding the bubbles, the CPU drops, but maybe not the memory.
Of course, it isn't quite this simple :-)