Nothing is future proof. The closest you will get to that from Intel (who love a socket or twelve) is their E platform, which tends to stick around far longer than one of their disposable desktop sockets.
Skylake isn't future proof as Intel likes to change socket with almost each new cpu. Only things that could justify Skylake over Haswell could be Sata express or USB 3.1. But considering your mate wants a cheap CPU he will not get sata express SSD. Problem: There are still not much sata-express drives nor USB 3.1 devices and we do not know if there'll be much ... remember Firewire and Thunderbolt. Why won't he stick with ITX ? Does he plan more than 1 GPU ? Skylake allows for 8GB per RAM stick, so he could even get 16GB on 2x DIMM, should he even need that much.
X99 isn't exactly perfect either with it's flakey memory controller, ultimately you're looking to get the best current gen bang for your buck. Buying from new it would be daft to go for an older generation so called CONSUMER GRADE PRODUCT.
Depending on the model of 1151 mobo depends on what ram it supports, what chipset is it? h110, h170, z170? Also you need z170 to use DDR4 speeds over 2133MHz iirc.
h170 price wise is good but z170 appealing because of higher speed ram! ITX is out for him. He is his own boss. For me, i love ITX. Dont need anything bigger case wise.
Higher speed RAM won't make any meaningful difference. If he's not overclocking then there's no need to go with Z170 imo.
i3 6100 with 2133MHz - i3 6100 with 2666MHz ram roughly 12 FPS difference in some tests. That 12fps could be the difference needed to hit 60fps.
Really? Using integrated graphics, I presume? Got a link for this, as everything I've seen suggests that RAM speed makes no real difference if using an external GPU. Bear in mind that the chap in the OP has a 960.
This is the video I watched. It was using a Titan X so it was a true test of the CPU. I'm at a point in the video where the difference is 10fps between 2133 and 2666, I could of sworn there's a point in this video where it's 12fps. https://www.youtube.com/watch?v=G5ejBlynOV8
