I think it's a chipset limitation but I'm quite sure that motherboard won't support more than 4GB RAM. You best option might be to install the single 4GB DIMM.
Thanks for your answer. A few more questions
1. What will run faster 2X2GB or 1X4GB?...
2. One of my RAM stick(2GB) still works. DDR2 RAM is really expensive in India. I was thinking of buying another 2GB stick.
3. Since MB is limited to 4GB what is the advantage and point of having 1 slot empty?...