8MB is even below the minimal system specs for a graphics card.
It does take more memory than having it off, but it takes less time to render.
It does not use the Med and Low folders, otherwise it wouldn't be called mipmaps, mipmaps is something that's done on the graphics card itself, not the CPU.
So, on a fast computer, you won't notice anything.
But your 8MB graphics card mostlikely won't even have mipmap ability. ;)