In DDR1 and DDR2 the differences are still small but show up in benchmarks whereas DDR3 only synthetic benchmarks.
In any case to a point more RAM is 'better' but only if there's no 'FREE' memory listed. Once 'FREE' memory is exhausted then the memory management begins to take 'Shared' memory which in turn can slow other functions from their optimized state. Also, there's Applications e.g. Rendering including Photos & Videos which do better with large amount of RAM.
So it depends on the OS, Applications, and Project Size as to how much RAM is recommended or need to run in its optimized state.
As a general rule my minimum recommendations for Windows XP = 2GB, Windows 7 & 8 = 4GB and double that for ideal minimums and Adobe & Rendering 16GB+.