Okay well if you are going mITX I'm gonna say you should just go Intel. Really, at that form factor an Intel CPU with a dedicated GPU would be the pest performing system.
COuld you use an APU and a dedicated GPU? Absolutely. But the performance won't be as consistent. Most games rely more on the power of the GPU. So theoretically you could throw a really expensive GPU in with a cheap CPU and game all day. The issue comes with games that use CPU power as well. Games like Crysis, Metro, Minecraft, DayZ, Primal Carnage, Natural Selection 2, as well as most games at 1440p+, use much more CPU power and the APU just can't keep up.
So you may be able to use a 280X and an A8 and play Bioshock all day at 1080 and have a similar experience as someone with an Intel i5 and the same 280X but when you go and fire up Crysis 3 his game is running at a smooth 60FPS and yours can hardly manage 20. The CPU (APU in this case) is bottlenecking the GPU so it runs slower and any processing that needs to be done by the CPU can't be done quickly enough hurting performance.
So yes you could use an A8 with a 280X, but it won't perform as well, usually, in most games (maybe a slight decrease) and in games that rely on the CPU more it will choke.
You want to build a balanced system.
You have a very good build there but it is impossible to recreate exactly the same performance in that case. You'd need to use an FX series CPU and there aren't any good mITX motherboards with an AM3+ socket.
For gaming 1866Mhz memory isn't necessary because when you get down to it, games don't need that kind of memory bandwidth for the CPU portion of game processing. Like I mentioned above games use the GPU more. The RAM on the GPU has all the texture files and everything that needs to be sent to the GPU for processing. That RAM needs to be fast. That is why GPUs have GDDR5 clocked at over 7000Mhz in some cases.
System RAM however doesn't store texture data or other large amounts of info that need to get to the CPU quickly. So you can go with slower RAM. This always seems to upset people, usually because they spend a huge amount of money on fast RAM, but in gaming you won't see any change in FPS between 800Mhz memory and 2600Mhz memory. Maybe 1/2 an FPS. You also have to remember that as the clock speed increases so does CAS latency. It is a trade off. Both are important in achieving high bandwidth. 1600MHz is usually the ideal balance between clock speed and latency.
So I could spend hundreds on expensive fast 1866 or 2133Mhz RAM and get the same performance as someone who spend $50 on some cheap 1333Mhz RAM. For the most part, RAM is RAM. You don't get much by spending hundreds of dollars on it, besides maybe a fancy usually useless heatspreader. That Corsair Vengence RAM is incredibly overpriced btw.
HOWEVER, if you are using an APU platform and relying on the integrated GPU on the chip you need fast RAM. System RAM in that case serves the same purpose as dedicated video RAM on a GPU. It holds textures and other important information that the GPU needs quickly. So on an APU platform where you are using the integrated GPU you need fast RAM because it also serves as the video memory for the onboard GPU. In that case the difference between 1600Mhz RAM and 2133Mhz RAM maybe a difference of over 15FPS in game.
Sorry about this massive wall of text. I wanted to try and explain it as best I could.