Can any one suggest me, how to determine the initial SGA size. Some documents are suggesting that SGA should be 1/3-1/2 of the physical memory
1/3? Which document. Some people will tell you that the SGA should be 50-60% of the RAM. I would say depends on the system. You will be on the safe side with these numbers but you are also wasiting memory.

As I see your SGA is a bit less than 1G. You say you have 2G RAM. Why waist it?