Prabhakar Chauhan :
Ok thanks you all for helping me...
Windows 32-bit can only "talk" to what's called a memory map and that is 4GB in TOTAL.
This includes not only the memory sticks (4GB) that you attached but also the VIDEO MEMORY as well as some minor things.
It sounds then that you have 512MB if VRAM (video memory). When the computer comes up it assigns this 512MB to the memory map, the other hardware resources and then what is left over from the 4GB memory map can be addressed.
If you'd had 2GB only then it would ALL have been utilized since you wouldn't exceed what is addressable in the map.