Does the read/write speed factor in when playing games
Yes, and that is most notable during loading screens.
E.g when you have your game stored on a HDD, it will takes ages during loading screens since PC needs access to the game files to load another part of the game. And HDD being the slowest in regards of read/write times, it will take long time to load as well. SATA SSDs are up to 10 times faster than HDDs and most consumers are happy with SATA SSD. M.2 NVMe SSDs are up to 6 times faster than SATA SSDs and compared to the SATA SSDs, game loading times differ only few seconds.
Here is a good video showcasing the read/write speeds between HDD and SATA SSD;
View: https://www.youtube.com/watch?v=j84eEjP-RL4
will the Game exectables be stored on this NVMe?
By default, new games are installed on the OS drive (C: ). However, you can change that within Steam settings.
For example, in my Haswell build (full specs with pics in my sig), i have Steam library and all games installed on D: drive since i like to keep games and OS on separate drives on that PC. On my main PC, Skylake (full specs also in my sig), i have Steam library installed on C: drive (960 Evo) since i don't install plethora of games at once.
If games are loaded here, would it be beneficial to get a 1.5 or 2 TB SSD?
1.5 and 2TB drives make sense only when you plan to install ALL or most of your Steam games on your storage drive and also keep all your pics/vids there. It's up to you how many games to install but i prefer to install few most played games and once i'm done with them, uninstall them and move to the next one.
For example: my Skylake build OS drive is 960 Evo in 500GB in size. Besides OS, i also have about 30 Steam games installed there (most games are small, casual ones). But since i keep my pics/vids/music on another drive, i have 169GB free on my OS drive.
What about when I plug in a Nvidia 3000 series equivalent GPU sometime in the future?
While M.2 NVMe SSD does use PCI-E lanes, up to 4x lanes, it isn't an issue since GPUs use 16x PCI-E lanes and if you're going with 3rd gen Ryzen CPU (e.g R5 3600) then those CPUs have 24 PCI-E lanes. Moreover, X570 chipset MoBo itself also supplies up to 16x additional PCI-E lanes. With it, you're looking towards up to 40x PCI-E lanes in your system, which is more than enough for your use.
Will a quicker read/write factor of the system drive matter then?
It does matter, especially during PC boot up and shut down since you don't have to wait that long. Look the video above to see the difference.
A 1TB 100 dollars NVMe or an almost 200 dollars one for the same performance.
Cheap price isn't the only factor between M.2 NVMe SSDs. If it were, no-one would buy Samsung drives. But for some reason, Samsung rules the M.2 NVMe drive market. So, there must be something else why people are paying extra for Samsung drives.
WD SN550 is an average M.2 NVMe SSD, it isn't the worst and it isn't the best either, when it comes to the performance.
Working against the WD Blue, Intel’s SSD 665p and Crucial’s P1 are both very competitive when it comes to light workloads and respond quickly to applications. But with their QLC NAND, the performance of those drives can sometimes lag behind other options. The same goes for the DRAMless Team Group MP33. With larger dynamic write caches, these drives can keep up with most other NVMe SSDs on a day to day basis, but once they are pushed beyond their cache, performance suffers greatly.
Source, SN550 review:
https://www.tomshardware.com/reviews/wd-blue-sn550-m2-nvme-ssd-review-best-dramless-ssd-yet/4
It's up to every person to decide if they are happy with average product (SN550) or if they are happy with best product (970 Evo).