Yes, this feature is support for pcie gen 4, but atm if I’m correct it only matters for storage speeds.
1660 and 3700x wouldn’t be well balanced build imo.
Like I mentioned before 3700x will only benefit you if you gonna utilise this extra core and threads , and in games you’ll not. Even r5 3600 will last you for quite some time till you gonna need to change it. And FX chips wasn’t.... great saying this softly nothing comparing to Ryzen chips