It is one of the things I do on autopilot and only came back into awareness again today when I was fixing up someone else's computer.
If you are running an AMD APU without a separate graphics card you might be satisfied with the fact that after you have built the system, and push the power button, it runs.
You might have gone into the BIOS to configure the Boot priority of your drive and perchance set the RAM timings.
You then game on the machine and the results are somewhat underwhelming. You then go on YouTube and tell everyone how pathetic the 7870K is for gaming (or any other AMD APU you might be using).
The reason for this is that you have omitted one MAJOR step along the way.
The thing is that the GPU portion of the APU needs RAM to perform optimally. One thing I can guarantee that most of you have forgotten to do is, in the BIOS, to choose the onboard GPU as the default and what is worse you have forgotten to assign it the maximum amount of RAM possible - in my case with the Gigabyte F2A88XN-WiFi it is 2GB. Leaving it at the default will mean that the graphics performance will be disappointing.
For this reason the minimum amount of RAM you should be considering for your system with a 7870K is 8GB. Anything less than this and you are kicking yourself in the nuts with regard to system performance.
It does not stop there however. With RAM good motherboards will allow you to do what is known as Bank and also Rank Interleaving.
So let's say you have a motherboard which has four RAM slots, those RAM slots will be separated into two Banks and if you have only two DIMMS you will put one in Bank 0 and one in Bank 1. What happens is that when the system is running the banks will be accessed alternately so that the refractory period which the RAM needs until it is able to be accessed again is effectively halved because the workload is spread between two separate channels.
If however you have four DIMMS installed you can activate Rank Interleaving where not only is the workload spread between the two banks but between the two ranks within those banks, this also will result in a performance boost for your RAM.
This doesn't mean that Bank Interleaving will double the speed of your RAM nor will Rank Interleaving then quadruple the speed, it will however boost the speed at which the RAM can be accessed significantly.
So if you are considering getting something like the 7870K and don't want to have an extra GPU and you want 16GB of RAM then if you have a two DIMM mobo (as most ITX boards do) then get two 8GB DIMMs. If you have a four DIMM board then instead of getting two 8GB, if your board supports Rank Interleaving as well as Bank Interleaving, get four 4GB DIMMS, to get the most performance out of your system.