Most of the gains have gone towards conserving power vs increasing performance. Doubling the transistors per chip can give you ~2x the speed, but that requires 4x the power (which means heat you have to get rid of). Alternatively, you can get the "same" speed but use less power by using multicore systems (but this requires programmers to be able to take advantage of parallel processors).

Edit: For memory specifically, there's also the issue that at a certain point, adding more RAM doesn't help because the issue isn't the number of transistors but rather how long it takes to access the memory.