16GB RAM is enough for games, unless you also edit videos ... You can always add more later ...
With regards to power supply ... if you can afford a Platinum then get it ... higher end power supplies consume less power ... minimum would be Gold+ ...
Samsung Evo Plus 500GB is good ... Operating System files eventually get larger ... Well, you could get 250GB now and just upgrade after a few years ...
Consider the RX 5700XT also ...