AMD is coming out with it's next gen of CPUs in a month or so, possibly with a 5% bump in clock speeds, and a 10% jump in IPC....
Currently, the 3900X offers next to nothing over/above the 3700X, so, IMO, you'd be paying extra for unused cores/threads...
I'd either wait, or get a 3600/3700...
(No one can give a 5 year 'you'll be fine assurance, but, you'd 'probably be fine'....or I'll refund your money!)
