For the 8K as well as the 5K model the recommended GPU is actually just a GTX 1070.
The technical details would be a little much, but I'll try to explain it as short as possible.
Pimax 8K display resolution: 2x3840x2160 (equals 7680x2160)
Pimax 5K display resolution: 2x2560x1440 (equals 5120x1440)
Actually rendered image resolution for both HMDs due to limitations with Display Port bandwidth: 2x560x1440
The 5K will display those natively and the 8K has an upscaler built in to scale the image to 2x3840x2160 pixels.
Even though it's a bit disappointing to have an upscaled image on the 8K HMD, I'm sure it will look a lot better than on the 5K model due to the higher pixel count (which equals less screen-door effect).
Waiting is a good option here. I'm sure the HMD will hit the market, since the product is already finished and has been shown at expositions within the last months. The company even already made a VR headset that is available on Amazon (the Pimax 4K which sadly does not support lighthouse tracking).
However, I'd also like to see myself, how good or bad the upscaling actually affects the image quality, before I shell out my money for it (even though I'd only pay for the HMD itself, since I can reuse my Vive's tracking solution).
And now for something unrelated: If you got an HTC Vive, you should really play To The Top. That game's awesome.