Very good advice!
My only additions are that the RX580 is probably the card to go for at this stage given that there will be native support for it in the next point release of Sierra, and in High Sierra when released. (No need to modify .kext files)
You probably won't have to wait more than a few weeks for 10.12.6 as it is approaching release. It'll probably take you this long to source an RX580 anyway as they are in short supply! The model linked below is the one Apple includes in it's eGPU dev kit so will be fully supported, including listing of model number in 'About This Mac'. (Most other cards don't have this without some 'hacking'.)
SAPPHIRE Pulse Radeon RX 580 8G GDDR5 Dual HDMI/DVI-D/Dual DP Graphics Card - Black
Finally, my SSD recommendation tends to be Kingston's HyperX Predator PCIe AHCI SSD. SATA really is old tech now and we should look forward from it.
HyperX Predator PCIe Gen2 x4 HHHL (Half Height Half Length) Internal SSD Drive - 480 GB
Driver support may be there, but boot screen support still will not. And it will not be macEFI flashable. I know a lot of people write off boot screen support as not very important, but I find this baffling (for one, how can one care about being listed properly in the "About this Mac" dialog but not care about a proper boot screen?
)
I myself use single user mode, verbose boot, and the startup manager very frequently, but I understand this is not common for most people... But don't people use Filevault? C'mon people, we're almost 2 decades into the 21st century already!
Well, if you do care about having a functioning boot screen (as you should) but also want a more modern, more powerful GPU, you may be able to get around the issue by acquiring an old GT120 and sticking it in slot 2 for when you need the boot screen, and install the RX580 in slot 1. A bit cumbersome, but it will help if you have a display that supports multiple inputs so you can connect it to both cards and just switch between the inputs as needed.
And while it's true that SATA - which predates the SSD era - is old and slow compared to newer PCIe and M.2 based flash storage, it's also sadly true that, after being around for nearly a decade, the architecture of these classic Mac Pros is starting to show its age and there will be bottlenecks in throughput due to being limited to PCIe 2.0 and slower FSB speeds which will prevent the fastest PCIe flash storage from reaching maximum theoretical throughput.
As such, these new-fangled dedicated PCIe SSD boards are not going to perform any faster on our classic Mac Pros than SSDs connected to a 6GB/s SATA or SAS PCIe card, especially if they are in a RAID 0 configuration. And you'll get much more versatility with SATA SSDs than PCIe SSDs, allowing you to upgrade and swap drives between computers at will without worrying about proprietary SSD blades or using up additional PCIe slots when you want to add more storage.
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570/580 at serious rip off prices at the moment. More than a GTX1070 and much weaker! And so now it's pushing up the prices of the 1070, which then pushes up the prices of the 1080.
This is what happened when alternate facts/coins/reality hit the computing world. We have to pay the price for the crazies.
You are better off buying a used RX 400 series from someone who is honest about pricing relative to actual RRP.
It's happened before with bitcoin. Then dedicated mining machines came around that made using off the shelf ATI/AMD GPUs cost ineffective, allowing those GPUs to come back down to reasonable prices again.
This current spike, if it can verifiably be traced to a rush by amateur coin miners, is just due to Ethereium suddenly hitting peoples' radar in the past couple of months. Dedicated miners will soon be crowding that field too, rendering off the shelf GPUs unattractive for this use case.
Point being, it's not the "crazies" driving up the cost, it's the profiteers and opportunists (sidenote: with this being a Mac enthusiast forum, you really ought not to use the term "crazies" disparagingly. This is supposed to be a place of refuge for the crazies. The misfits...)
Also, Vega GPUs will be arriving soon, further driving down the cost of the 400 and 500 series. And they will be releasing dedicated GPGPUs for the people who need them for HPC / computational purposes (or mining, as it were) which will leave ample supply of regular GPUs for those of us who need them to output to a display.
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If you worry about RAM performance, you should not go 8x16GB (128GB), but only 6x16GB (96GB). The CPU is triple channel, install 8 sticks will make them run slower. Anyway, the difference is very significant in benchmark, but I never heard anyone can actually tell the difference in real world.
I was under the impression that the speed penalty for filling the 4th slot was relatively minor since it supposedly only slowed down slot 3 (and was still faster than foregoing the extra ram requiring more swaps to disk), but that the penalty for mismatched RAM - especially across dual processor variants - was significant (hence the warning that OS X gives when mismatched memory is first installed). But maybe it's more significant than I thought? Or maybe neither penalty is that bad compared to the benefits of more ram?
Oh well. Like you said, probably too irrelevant to make a difference in normal real world usage
