I'd disagree. The days of CPUs doubling in speed every 1.5 years ended around 2000. The speedup since then has only been about 5%-10% per generation (roughly 3 years by Intel's tick-tock cycle). So at the same clock speed, the speed difference between Sandy Bridge and Skylake (2 architecture improvements) is only going to be around 10%-25%. Not worth it.
The integrated graphics sees much more improvement with every iteration, but that's irrelevant since you're using a dedicated GPU. Aside from that, the only practical reason to upgrade to Skylake is if you need USB 3.1 support.
8 GB of RAM is still plenty unless you're using virtual machines, doing heavy multitasking, or doing heavy video editing. As the future looks to be DDR4 memory, you'll probably want to do a full system upgrade if you wish to move past 8 GB.
The RX480 still holds up fairly well against the 1070. So you can probably wait another gen to upgrade the GPU.
I'd add a SSD. 250GB or larger, to hold your OS, apps, and 1 or 2 of your most-played games. 500GB if you want to be lazy and keep your games and data on it. Be aware that SSDs operate best with about 15% free space, so don't plan on filling it up completely. The Samsung 850 EVO series is practically indistinguishable while in use from the Pro, while remaining very cost-competitive.
Intel/Micron have been making lots of noise about 3D Xpoint being released at the end of 2016, so you may want to wait on the SSD a couple months to see how that shakes up the storage market.