What is CAS latency in RAM (buying advice needed)

I am going to purchase new Ram for my comptuer, I have currently 2GB x 2 DD3 ram and now going to buy 4Gx2 RAM, that will make total 8GB RAM in board. Unfortunately my board has only 2 slots so to go higher than 8GB, I will need new board or new modules but that is for another time.

now I am in dilemma which one to buy , I see one transcend with latency 9-9-9 and one from G-skill 7-7-7 , so which one is better. There is Rs. 500 difference in both ie. later one is costlier. Should I buy 9-9-9 latency ram or 7-7-7 ?

There is one simple rule, lower latency means faster RAM but in real life you will hardly feel any difference, you can only notice difference using some benchmarking softwares / tools. Going for lower latency RAM wont hurt but if you can spend Rs. 500 extra per stick then go for second option ie. Gskill with cas latency of 7-7-7