You should be just fine. Two days ago I came in on 889 and transferred to Air China to Shenzhen, scheduled for 1735pm. You arrive in Terminal 3 and depart from Terminal 3 (Air China is now completely in Terminal 3), so no problem.

Came in and got off the plane. Walked to immigration, lines not bad at all and the speed of processing was good. Then go down to a train to get to the other side of Terminal 3 (it's loooong), there are signs that say don't worry, trains arrive every 3 min.

Get off the train, and pick up your luggage. Then follow the signs for domestic transfer and check in for Air China. Sure, UA gives you a boarding pass, but here you check in your luggage, plus you can change your seat if you want (I did, because UA couldn't). If you are 1P or greater, feel free to use the 1st class counter, since it says "Star Gold." They'll tag your bag with Priority. And it does work.

From there, go through security (laptops out, liquids out, shoes can stay on), and then find your gate. Making a 1640 flight should be no problem at all based on my experience.

Boarding Air China, if you are 1P+, they will let you board with 1st class, in a priority lane. My pass didn't say 1P or anything, so I had my card out just in case. It was nice to bypass the crowd for general boarding.