How important is RAM speed for computationally expensive tasks like gaming, video encoding, video editing, graphics software (Photoshop/Sketchup)?
Solution:
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.