thank you for your reply
But my motherboard supports up to 1GB
why 128 mb
Most games require 512 or higher
In the event you didn't understand, that's the minimum assigned/reserved to the GPU and it will take more as needed automatically up to the max allowable, like when gaming. If you dedicate like 1GB outright you are literally wasting ram when doing non graphics intensive stuff. Therefore it is usually better to leave it as is.
Also if you have a dedicated GPU like an nVidia/AMD then it will be using the graphics card's RAM first before trying to reach system memory.