I wrote this in another post but here is an example of the different between HDD, SSD, NVMe
an HDD you can get around 100MB Read correct? and a SSD averages aroud 550MB and then then 950 is 2500 MB
So the SSD is about 5 times faster than the HDD and then NVMe is about 5 times faster than the SSD. But here is the thing. A lot of times you are loading small files when windows or programs loads. Games you don't see as huge of a difference past SSD.
So if a 1GB file takes 10 seconds to load on a HDD and it takes 2 seconds on a SSD that is a whole 8 seconds? that saves a lot of time right!
Now that same 1GB takes .2 seconds VS 2 Seconds. Now count to yourself. 1 Mississippi, 2 mississippi. 2 second is still not that long? and in most cases a lot of files are super small, few KB to MB. You aren't loading a whole lot of apps that have files greater than a few KB/MB which even with a SSD is too fast for us to really count. So in the end for basic every day stuff you will not see as big of a difference between a SSD and a NVMe. Better to save your money and get a single big SSD and have everything faster vs just a few things.
Now if this was a server, doing video rendering, SQL, some kind of huge compiling then Yes NVMe would be the way to go but for everyday/gaming use it really isn't worth the extra money.
SO how much faster would it be? Not much. Here is a video that compares them.
https://www.youtube.com/watch?v=kxYGM-MlngU
yes the timing is a little off and it is the intel 750 which is similar in speed. but as you can see in boot time they are pretty much the same! Because windows loads so fast as it is already you don't see much of the benifits of NVMe.
If you want to get a 950 pro and still use it on the PCIe 2.0 Go for it. You won't see any difference either way. If it was me. I'd just buy a 850 Evo/Pro and save money.