You're right. I may have exaggerated a tad, based on my experience of seeing the gtx960 outperform literally every AMD card in a benchmark a few years back. Still, there is a gap, and in most situations Nvidia performance difference between Linux and Windows is smaller than AMD. Those  recent 480x benchmarks give me some hope, though. It might be worth it jumping ship soon. AMD is much more attractive to me as a company, and their commitment to open standards is commendable.

As for open source, I figured that anyone gaming in Linux had pretty much already resigned themselves to using closed source software in some capacity.

When it comes to PC's everything is upgradable honestly. 

The thing to know is that Graphics cards are usally the easiest to upgrade and give you the most performance between upgrades. The downside is that they become obsolete much faster than the rest of the PC. Plan on a 2-3 year upgrade schedule depending on how much you intially pay into it.

• Low end price: 150-250
• Mid Price:         250-400
• Hi-end Price:    400+

CPU's can be upgraded but often have to be upgraded at the same time as the motherboard as architectures change. The reason behind this is that CPU upgrades are often not worth it unless you buy a really cheap chip with plans to buy a higher end version when you have the money. If you buy at least a mid-grade chip it will last you 5 years before it becoes obsolete. 

• Low end price: CPU: 70-150     Mobo: 50-125
• Mid Price:         CPU: 150-400   Mobo: 125-200
• Hi-end Price:    CPU: 400+         Mobo: 200+        

Right now the Memory sweet spot is 16GB but if you are buying now i'd suggest 32GB. Memory is tied for the second longest lasting part in your PC and can last long enough that you can use it with two sets of CPU/Motherboard upgrades. I personally used the same set of DD3 sticks (8GB) for nearly 6 years and with two differnt motherboard/cpu compbos. 

• Low end price: 16GB: 45-75
• Mid Price:         16GB: 75-125
• Hi-end Price:    16GB: 125+

The part that is tied with the Memory for longest lasting part is your system storage, aka: SSD or HDD. Best suggstion here is a 512GB SSD and somewhere between 1-3TB's on an HDD. This combo will easily last you years and years. Just make sure you also buy hard drive that is at least the same total size as your SSD and HDD so that you have one for backups. Unless you are running programs like Photoshop all day long you should just stick to decen non-performance based drives. It is a waste of money otherwise.

• Non-performance SSD Price: 256GB: 125-150
• Non-performance HDD Price: 1TB: 75-125

Finally the longest lasting part for you PC would your Power Supply. Buy a good 650W Silver or higher 80 PLUS rating power supply and it will outlast every other part in your system. Never go cheap on your PSU, if you do and have a failure it can fry the rest of your PC. Go with a strong name brand and 80 PLUS  silver as a minimum.

• 650W 80 PLUS Silver price: 90-125
• 650W 80PLUS Gold price: 115-200

What's the difference?

@Jemc: Starting January the earliest. I have to buy components only a couple at a time for few months, as I have my car to fix at the same time (cylinder head cracked and turbo leaks oil, it's likely going to get a minor power upgrade from 130hp to ~200hp as well as I fix it).

@Raistline: Thank you! Very useful information. I was thinking as fast as possible 32/64 or so GB SSD as a root/home and HDD for everything else. That's because how fast the SSD prices are (hopefully) going to drop - though, 32GB is way too small if I have that much RAM (although, planning on swappiness=0 anyway).
It's likely that I upgrade the OS every two years (Ubuntu LTS), and I've liked it clean, so this is where I could change the SSD if I need to.

Basically, parts that I need to upgrade, I can also reuse on another computer, as I have quite a few kids who are likely going to want their own computers too at some point.

The reason I was thinking the upgradeability, is because I learned some time ago that laptops aren't necessarily upgradeable. My netbook (wife's netbook actually) RAM maxes out at 2GB, CPU cannot be changed, and if it could be, the socket wouldn't fit anything even remotedly modern.

Apples and Linuxes..

touché

Maybe tell your budget range first?

If you're going to buy it in parts, I would start with the components with stable prices first, like the case, PSU and RAM, then the ones that don't change too fast like SSD/HDDs, motherboard and CPU and lastly the GPU.

That should allow you to see the impact of the new Intel and AMD processors and the launch of AMD's Vega GPUs in the market.

Here

By him a Macbook instead. He'll be the coolest kid on the block. lol