Bram387 :
I am building a 4k gaming pc with a gtx 1080 which cpu and motherboard do you recomend want to upgrade it to sli after a couple of months
If you are going to exploit SLI to it's fullest, I'd recommend a Z170 motherboard with a PLX chip, which allows for a 16x/16x PCI-e lane GPU config coupled with an 1151 Skylake CPU (6600K will do fine and can be OC'd). Off the top of my head, EVGA and Gigabyte may be the only manufacturers that do this, within the Z170 line.
Some will state that there's not much difference between 16x/16x and 16x/8x SLI. But note that one word... Difference. So, it does matter as more GPU data transfer translates into a higher FPS count. Even if only a few performance gains from this, take them as these PLX chipped motherboards are not astronomically priced.
Depending on what else you utilize your PC for, i.e. heavy video edits, multitasking (beyond the norm) a X99 may be a better option than Skylake. Intel just released a new line of CPUs (Broadwell-E; of these the 6800K or 6850K would be plenty good/least expensive) and drop into a 2011-v3 motherboard for even better multi-GPU support; faster GPU lanes/data transfer speed(s). Going this route however, depending on what you choose (CPU and motherboard models), can also be far more costly to your budget vs. the Skylake option.
Also, for 4K gaming... Even the "mighty" GTX 1080 as it stands with current offerings would require a 2-way SLI setup to reach 50-60+ FPS range with settings maxed/ultra; newer factory OC models that are coming will increase performance numbers but will also incur a premium cost.
4K is a VERY demanding resolution... For me, getting 30-mid 40's FPS in games at 4K or any resolution is not only unacceptable, it is laughable but others are happy with it. To each their own... But If going that high on pixel-count for a pretty picture why not have FPS to go with it? Just my opinion here.
Hope this answers your question(s) to some degree.