Would really depend on the game, some love fast memory (i.e. Fallout 4) others not so much. The current trends in games are going to faster and more DRAM as the developers of games are utilizing DRAM more and more
it makes a difference for sure. but the biggest difference for most people is the fact that the move to ddr4 involves a move to a stronger skylake i5 over whatever they are replacing.
if you have a ddr3 and 4 capable mobo, then the ddr4 ram will give a little bit more performance. not sure of exact fps but it will make a difference. seen some tests of slow vs fast ddr4 ram and it made a measurable difference so i'd assume a move from ddr3 v ddr4 would be even better.
Tests I have seen show no difference in performance between ddr3 and ddr4.
But, ddr3 from older gen builds will not work on skylake because they generally exceed the 1.35 voltage max.
New ram kits cost the same between ddr3 and ddr4.
Would really depend on the game, some love fast memory (i.e. Fallout 4) others not so much. The current trends in games are going to faster and more DRAM as the developers of games are utilizing DRAM more and more
Programs that deal with large chunks of data benefit from DDR4. These would include image editors like Photoshop, video editors like Premiere, 3D graphics apps like 3DS Max and Blender, and of course file compression apps like WinRAR and 7zip to name a few.