New PC build - Skylake and Windows 7?

No reason to do that, win7 is fine enough

DX12, you only get that with Windows 10. Also, Microsoft is introducing a new "gaming mode" for Windows 10. Apparently it will detect when you are playing a game and throttle back on background processes, things like that to help performance.

I highly suggest moving to 10. I know some will hold on to Windows 7 until the bitter end, and there are many people still using XP, but 10 is not that bad. Yes, you do get forced updates, but for most that just takes the hassle out of checking for updates. It is no more intrusive than the way Firefox handles updates, for example, or many smartphones. It's the way of the future.

soon u can pause windows 10 updates for 30 days

DX12.
Much faster boot time (in extreme cases, almost one-tenth of Win7 boot time, but it is a Win8.x thing as well).
The fact that the kernel can't just randomly decide to not stop working (unless you are in the Insider program, using beta OS).
A much better explorer.exe that automatically resets itself when crashes (again, Win8.x feature).
A process explorer that can actually be used for something (ditto since Win 8.x).
An update method that won't turn your machine unresponsive and unusable until it is finished. (I honestly cannot even tell when my OS updates, only when the shutdown command suddenly changes.)
The option to choose to never install Flash player ever again; if you really need to use it in 2017 for some ungodly reason, it is built-in to Edge and managed by MS itself (so if something is patched, they take care of the update fast).
A lot less resources used. (It may not be too important for you with a Skylake chip, but I use an ASrock motherboard and I like that my CPU runs at 0.77 GHz when it is under light or no load, drawing a lot less power. Win7 in idle was more demanding.)

You also get a UI that is a weird mixture of Metro and classic (I like it, but it is weird), and the strange decision to divide the Control Panel into two separate things. Also, the need to manually force the OS to not try to auto-update your drivers; especially on AMD, where it simply uninstalls Catalyst but never actually installs the new version.
The need to fiddle around for an hour or so to set up the user interface in a way that it is not intrusive as all hell.
Also, the forced updates are annoying, but not absolutely deal-breaking. Pro/Enterprise is significantly better though.

As for compatibility, right now it is not worse or better than Win7 after its first year; most anything runs well enough.

Ok, so it's possible to still install Win 7 but I won't necessarily get the best performance.

Can anyone clarify the difference between i5-7xxx series and i7-6xxx series? I understand i7 is Kabylake and only supports Win 10.

You can install Windows 7 and use it for as long as you want. I am still using Windows XP on a fairly new computer even though mainstream support ended in 2009 and the extended support ended in 2014.

No reason to use Win 7 whatsoever. Seriously. Especially with a machine like that.

I bought recently i5-6500 (new 7 series is just ~200MHz faster) and it's damn good. :) And You can use win 7/10 but only win10 supports directx12.

https://support.microsoft.com/en-us/help/13853/windows-lifecycle-fact-sheet
If you want to avoid Win 10 you can keep Win 7, but updates will stop, depends on you how important are updates to you. Personally i wouldn't worry for a a good while specially if you are happy on Win 7.
You can also install Win 8.1 and get +3 years of support, plus most speed improvements except dx12.
You can keep using Win 7 for now and install a newer windows when you feel like it, a newer build shouldn't have problems on Win 10 but sometimes upgrading can be not that smooth and get issues here and there, in that case a clean install would be preferable.

Thanks guys, I appreciate the input. I feel happier to go for an i5 6600 running Windows 7 until I feel the need for updates and then I'll migrate to Windows 10 when hopefully they will have changed the user settings for automated updates. (Not likely I guess).

For Windows 10, you can stop update this way

• Open Windows Setting
• Search View Local Services
• Select Windows Update
• Change Startup Type to Disabled
• Select Stop on Service Status
• Apply and OK

fwiw, I built a new PC this summer (finally upgraded from my old XP machine), and have had no issues with Windows 10 and actually like it quite a bit. (I went w/ i7 and Nvidea video, so can't speak to any issues around AMD/Radeon).

i would go with a i7 so that you have Hyperthreading and just bite the bullet and go with Win10 and use a some free programs to turn off its obtrusiveness like Shutup10.

Got my new PC recently, had Win7 before but I did change to Win10 when upgrading PC and I'm happy with it. Got 6600k currently OC'd to 4,5 GHZ stable :) Also u can turn off updates in windows settings.