RAM speed: How does it impact performance?

For video encoding & editing : Marginal improvement, as the hard disk is the real bottleneck, not RAM.

Graphics software : Marginal improvement, and only on some very rare operations. Format conversions will improve, but not very noticeable unless you are converting batches of images.

Gaming: Depends on the game, but usually here is where you might find some real improvement.

It should be one of the lowest priorities when choosing where to spend your money because the speed increase / $ is one of the lowest. It is usually better to expend the money on a faster processor, a better GPU or more memory.

It all depends on what you are using your PC for. If it's for applications that load completely into memory and run then quantity and speed of RAM are important. If, however, your application is constantly performing disk writes then you should look at the HDD access times as the greatest performance bottleneck. Different games will require different things, i.e. If you are playing games with AI then you need good RAM and lots of processing power, games that are graphics heavy will lean toward a good GPU/memory/processor, etc.

It really does depend on the individual application/game.