You might want to look up "AMD Zerotech" as its that way right now if you buy an AMD APU/CPU and pair it with an AMD GPU. With Zerotech if you use that particular setup it will turn off the discrete when not needed giving you the lower power usage of the APU and will then fire up the discrete when you have a task the APU cannot handle. Likewise if you have an AMD CPU it will turn down the GPU when not needed and simply use the GPUs baked in video encoder/decoder along with the frame buffer while turning the rest off.
I have an AMD FX-8320E paired with an R9 280 and I'm currently only pulling 8w from the CPU with 5 tabs and a video running and the GPU has idled down to practically nothing (my gauges only go down to 300/150 on GPU/memory speed so I cannot tell you how slow its actually running) with the GPU completely cool to the touch and the entire system completely silent.
I have to say I'm deliriously happy with the performance and power usage of my setup, last time I put it on my Kill-A-Watt it was pulling less than the Q6600 that I had been using at the shop as an office box, and thanks to Zerotech while the system stays nice and frosty and sips power when I'm just surfing and watching vids it can still scale up to 4Ghz on the CPU and 940/1250Mhz on the GPU/memory respectively in less than a second and if I want even more? I just flip on the overclock and can shoot up to 4.4Ghz without breaking a sweat. considering I paid less than $650 for this system with 16GB of RAM,a gaming board, 3TB of HDD, a BD burner, the R9 280 AND an SSD? The bang for the buck is just insane, no way you could build an Intel system with those specs, no way in hell.