Depends on the usage really. You do see a noticable improvement with things like editing and file compression where there's a lot of transfers going on between the CPU and memory, but otherwise it's not too large. Gaming benefits if you are using an intergrated GPU, but otherwise it makes little to no difference.
As for capacity, having too much will just result in it being unused. You'll see a large performance hit if you run out of memory, but that's generally not an issue if you have 8GB+.