linglingjr :
Thanks for the quick reply, my next question would be: what are the disadvantages of 1x8gb? I only ask because 2x4 is $9 more expensive
There is a post which has replied to this question you can refer that but anyway this is my version of the answer.
Your motherboard supports
Dual Channel Memory architecture and that's the reason why installing in a matched pair is preferred over single module. When you install the memory in matched pairs (in the correct slots) then the memory will run in the dual channel mode(automatically) and yes there is a performance difference between Dual channel and Single channel modes, usually around 10%-15%. That may not sound too much in reality but that's a better for performance.
Though you may ask if I run 4 x 2GB will it still in Dual channel mode? Yes, it will but I do not prefer this setup over the 2 x 4GB one. The reason being though both setups work fine the former (4 x 2GB) runs causing more stress to memory controller than the latter. Moreover, you do not have to ditch any memory if you are to upgrade in the future if you are going with 2 x 4GB but it is not the same with 4 x 2GB setup. In other words you will not have any room for upgrade without ditching some memory if you stick with 4 x 2GB.
Amount of memory: Anything between 4GB - 8GB should be enough to run most of the all the software's. But if you are running high memory intensive apps like Photoshop, audio encoding/video rendering especially when working with large data formats more memory will come in handy, 16GB is what I think. 32GB???
That's a big NO!! Unless you are using VM's.
Hope that answers it all. Cheers!!!
You can go through these links:
Memory:
http://www.memorystock.com/memory/GigabyteTechnologyGA990FXAUD3REV11MainboardDDR316001866MHz.html
Multi-Channel architecture:
http://www.intel.com/support/motherboards/desktop/sb/CS-011965.htm