If you get a DDR4 system, you'll future proof your system for a while. Graphics card performance changes drastically each year. 2160p is just getting the performance from hardware it needs now. You'll be seeing it at a lower price point as the generations continue, and thus you'll be seeing 2160p becoming the new standard for gaming by 2020. The GTX 770 you have is not capable of 2160p gaming, so it will need to be upgraded. Would I do that now? Not really. If you're looking to save money, wait on the graphics card.
In addition, you'll see prices on the 1080 drop when the 1080ti gets released. NVidia will likely launch it this year, before xmas, and probably before turkey day. The 1080ti will likely be around $700, maybe even $650 if AMD launches their challengers, which will push the 1080 closer to $500, and the 1070 to about tree fiddy.