Unless you have workloads that really shine with the extra threads - and I don't think your workloads do? - I can't see the move from 6C to 8C really paying back the extra expense.
As to upgrades, socket AM4 will last at least 2 more generations (next year in a move to 12nm with minor tweaks, and in 2019 with a major core revision and a further shrink to 7nm). I wouldn't consider getting a 1600 to upgrade to a 1700, but I would buy a 1600 now in the knowledge that there'll be a range of new processors in two years time that will drop in to the same motherboard!
iirc AMD said that Ryzen processors don't have a divider for 3000MHz RAM - they jump from 2933MHz to 3200MHz. On that basis - unless the price difference is significant - I'd be tempted to look at 3200MHz kits as a minimum...