1. DIR-850L is 2x2 ac router, so you won't be able to make use of the 3x3 capability of AC68.
2. If you're using PC with available free PCI-E slot, I recommend using PCI-E wireless adapter instead of USB. USB 2.0 & some of so-called USB 3.0 wifi adapter rely too much on your CPU & may have latency issues when your PC is on heavy load, which is bad for gaming.
3. If current 2.4GHz wifi signal detected by your pc is lower than about -60dB, you won't get high data rate with your ac wireless as 5GHz signal strength will be lower than that (5GHz had lower range than 2.4GHz at same power level, and had higher attenuation from walls blocking the signal) and require a good signal strength/quality in order to have good data rate.
4. My advise, don't so hung up on utilising the entire bandwidth by yourself especially on wifi. Realistically, you probably need additional access point if you want to make full use of the bandwidth across multitple wifi devices. If you are so keen on monopolising the entire bandwidth to yourself, then just use gigabit ethernet connection.