thx for the detailed information BonzaiDuck.
There's a gtx 1080ti in the 1st pcie 3.0 x16 and obviously this is the priority, i don't wanna compromise her speed for a faster booting OS. But you seem to say in your case it doesn't make any differences?
i would need something like that to fit the samsung NVMe ?
https://www.amazon.ca/Rivo-Riser-Ex...=UTF8&qid=1521478912&sr=8-9&keywords=ssd+nvme
mobo doc says x16, x8/x8 or x8x4x4
[I started my response here, having momentarily forgotten that you want to have a bootable NVME, and such might not be in the cards for you.]
I had looked into that question over the last year or so, about reducing PCIE lanes from 16 to 8 on a high-end graphics card. Various observations and bench tests I'd seen on this question suggest that any reduction in performance for the GTX 10xx cards is about 1%.
That's such a small margin you might wonder about error in measurement. The cards don't saturate the bus for only 8 lanes. If you overclock your graphics card as I do, it might matter even less.
I had been apprehensive that changing from 16 to 8 lanes for the graphics card would somehow ruin my overclock settings for the GTX 1070. It did not do so in any way. My WEI score shows 9.2 for graphics and 9.9 for gaming graphics respectively.
Again, given the prices, I'd recommend at least a 250GB NVME SSD. Truth be told, you can avoid adding further to your NVME drive, install a conventional SATA SSD, and begin to build "Program Files" folders on it, choosing it when you install new software. I have all my games on an SATA SSD which is cached to an NVME, but you don't need the caching -- my system was an experiment which turned out to work without flaw.
OF course, if you pursued the project of caching the second drive, a regular HDD would give you good gaming performance anyway.
[EDIT] Also, you could treat an unbootable NVME the way you might use the SATA SSD I mentioned above.
I think if you look on the internet, you will find procedures for editing the registry to move "Program Files" items from the boot-system disk to another disk. It just seemed a bit tedious to me.