As veladem said, the 4670k is the best CPU on the market. The 4770k is has little improvement over the 4670k in games as very few games actually use Hyperthreading. The 8350 is a good CPU from AMD but in order to keep up with the 4670k, you would need to overclock it.
With Mantle, the stronger the CPU you have, the less % performance increase there is. For example, if you have a 280x paired with an AMD APU, you could have a performance boost of 40% (that doesn't mean a boost on ultra settings. It means whatever the sustainable settings are (could be med, high etc)). If you have an 8350 or an 4670k, the boost will be less than 10%.
I do think if you can afford the price hike getting an R9 with a 4670k. However, the sweet spot right now is the 4670k and the 770GTX (for a budget).