Yes, the iGPU can be paired with a dedicated GPU in DirectX12 games, but it remains to be seen whether the performance gain is worthwhile. I kinda doubt it.
However the iGPU is used for other stuff. On mobile, you can disable the dedicated GPU and save a ton of battery juice when you're out and about. It's also useful for encoding h.264 video in hardware via intel quicksync.
Back to Ryzen, it is basically single-core competitive with Haswell/Broadwell i7s but not quite up to Skylake/Kaby, at the same clocks. On multi-core benchmarks it obviously obliterates everything with less than 8 cores. It doesn't overclock very well, so keep that in mind when looking at all the reviews. It won't get much over 4.0Ghz, while intel "K" CPUs overclock like demons.
If you're building a straight-up gaming battlestation, you still want an i7-7700k. Better IPC and it has a huge clock advantage if you overclock it to 4.8Ghz+. Otherwise, if you do stuff that benefits from more cores like content creation or code compiling, the R7-1700X is a very attractive choice-- reasonably close performance per-core to a modern intel i7, and you get twice as many of them.