The final rendering of an image in professional applications is a task quite indifferent to the speed of the memory subsystem. It is the CPU speed that plays the decisive role here.
When shadows are rendered in professional applications, the memory performance is more important, although the CPU speed is still the critical factor.
The algorithms lighting calculations are, on the contrary, much more dependent on the memory performance. We have Pentium 4 3.0GHz with DDR400 (2-2-2-5 timings) losing only to the Pentium 4 3.2 platform with the same memory.