So, I'm planning on building a PC for my oldest kid to play games with - although the OS will be Linux - and I'm not really sure what hardware should I get.

I'm looking for something that should be somewhat easy to upgrade in the future, so no motherboard from bargain bin that's compatible with outdated chipset only. Then again, if the best option is "ten times" as expensive as the second best one, I'm willing to go with the second best.

Also, the the amount of RAM the CPU/MoBo supports is important, as this is what I think is maybe the most crucial part in "future proof" hardware. 

From my experience, Intel CPU's work better with Linux than AMD, and the ATI/AMD GPU's seem to work better than Nvidia.

Suggestions? 

RAM is the cheapest way to upgrade a computer, with more RAM than you could possibly need costing no more than a few hundred dollars. Just remember to make sure that the RAM type is compatible with your motherboard (newer RAM sticks are DDR4 these days).

I'd recommend starting by investing in a good motherboard. I'm not sure the total RAM stick count is too important as they do tend to have rather powerful RAM sticks (16gb per stick is fairly affordable right now).

You seem to know what you're talking about though, and since you plan to use Linux I don't feel comfortable offering advice as I just go with windows to make gaming less problematic, though I understand Steam has been trying to make gaming on a Linux easier.

Get an i5 6600k or i7 6700k + Rx 480 8gb or wait for Zen/Vega in Q1 2017 if you want something more powerful but don't want Nvidia. Also put steam OS on there.

Just remember that an i5 + 1070 will give u better fps than i7 + 480. Idk about linux benches tho.

Get a z170 mobo and most if not all should have bios support for the 7000 series cpu.

Most motherboards will have the amount of ram listed on the specifications. For example

http://www.newegg.ca/Product/Product.aspx?Item=N82E16813132566&cm_re=asus_z170-_-13-132-566-_-Product

Supports up to 64gb of DDR4

I know it says $200 but thats also in cad so your milage will vary.

I cannot give you an honest suggestion for PC parts until you tell me what your budget is. From there I can give you the best suggestions that will put you as close to you budget as possible.

I can and even suggest a few areas where it would be advisable to upgrade if you can increase your budget by X dollars. For example if your budget puts you at a GTX 1050 range, I would also suggest that if you spen $30 more you could a GTX 1060 or ATI 480 and get 2x performance increase.

Nvidia cards work way, way, WAY better in Linux than AMD. Like, it's not even close, unless you're using the open source drivers for your GPU, in which case Nvidia is garbage. I'd really like to support AMD, but I'd be basically shooting myself in the foot. Check the benchmarks. Unless something drastic changed with this latest gen, you pretty much have just one option for Linux gaming: Nvidia.

System32 is not a thing on Linux. So if it were there and you deleted it, no gain or loss would be detected.

I haven't really thought about the budget. I'm not that into PC's to know which components have high value - and can be expected to be easilly upgraded a couple of years in the future.

I guess I've had bad luck with graphics cards, as the times I've had Nvidia card, I've had to use nouveau drivers, whereas AMD website has pretty good selection of drivers for Linux. But, thanks.

Nah, Apple and gaming would sound hipster.

No that would be retarted 

It's not that dramatic. The AMDGPU driver has made leaps and bounds and the difference with Catalyst isn't as big anymore. Nvidia needs closed source drivers to work and that's just silly on Linux. I game on linux with an old R7 265 and it works just fine.

When do you plan to build that PC?

Intel will launch their new CPUs this January, and that will mean a drop in price of the current 6xxx series (which are the ones to get as the new 7xxx series aren't worth it), and AMD will launch their Zen processors and compatible motherboards in Q1.

In any case, both platforms will use DDR4, so you can already buy that if you find a good deal. 16GB would be the sweet spot... at least in Windows.

Nvidia cards performed better than the AMD ones, but I don't know if that has changed much in recent times. The best card would depend on you budget.