Well I still don't use it so lemme rephrase: I can't tell you exactally seeing as how I don't use RivaTuner.
Aperature is the ammount of RAM your video card is allowed to use once it's own VRAM is maxed out (as DoomUK correctly pointed out). Generally 128MB is what I have heard the best option. And changing it will not give you better fps untill you have used up all your VRAM and are using your system memory and in which case I'm not sure of the performance there seeing as how I've never put my card under such a load.