I am getting a new PC soon, the motherobard has dual channel memory arcitechure. Which would be faster/better, 2x8gb ram with CAS latency 9 or 4x4gb with CAS latency 7?
It depends on the motherboard. If your motherboard supports more than 16GB of memory and has more than 2 slots then this answer qualifies:
Lower the CAS latency, higher the speed. So the choice would be to go with the lowest CAS latency possibly. But it also depends on the clock speed. Consider, the 2 x 8GB has a clock speed of 1600MHz and 4 x 4GB has a clock speed of 1333MHz then I would go with DDR3-1600MHz and the lowest latency available for the same speed which is CL8.
In the hindsight, 16GB of memory is more than enough to run for almost every software there is at present. So I do not think anymore memory is needed unless you are an overclocking enthusiast or perhaps you would like to upgrade for the sake of it. So I would go with the DDR3-1600 4 x 4GB and with the lowest CAS latency. (CL8, in case of this clock speed). But in the end, the choice is yours. When you need memory, maybe you might want to take a look at this site: http://www.memorystock.com/ or newegg.com