Is this a good mid tiered computer?

Yes, you have chosen quality components and as you say a good mid ranking build. You may find your SSD fills up quite quickly so you may need a conventional HDD to supplement it

Excellent build pending Windows, Audio output devices, keyboard, mouse.
I'm going to tempt you on $20 higher net after rebates.
http://pcpartpicker.com/p/n96gCJ
CPUAMD FX-8320 3.5GHz 8-Core Processor
- Because it is the same physical part. AMD has not yet tested it to 4GHz, but 99+% of the CPUs can go to the same frequencies. http://www.cpu-world.com/Compare_CPUs/AMD_FD8320FRW8KHK,AMD_FD8350FRW8KHK/
I need the $50 to go elsewhere.

G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-1866 Memory
The CPU memory controller is not officially supporting the 2133 ram. Although it generally works with no problem, the combo deal gets me $20 to use elsewhere. Given a 4GB Video ram dedicated, you should not be seeing improvements in 2133 on the graphics side, so it is only a slight CPU hit, if any, because of the controller in the CPU. You may get no difference at all, and it is approved speed ram.

PNY Optima 240GB 2.5" Solid State Drive
This also gets $35, and SSDs are only for data load improvement time. It is a small sacrifice.

EVGA GeForce GTX 970 4GB ACX 2.0 Video Card
This is the prize.
http://www.game-debate.com/...ce-gtx-760
http://www.anandtech.com/bench/product/1038?vs=1355
You also drop some few watts use.
Corsair Builder 500W 80+ Bronze Certified ATX Power Supply
Another $25 saved.
Watts: 145 GPU, 50 fans and drives, 130 OC CPU = 325 watt 12V gaming load
CX500 is still 456W @ 12V, an acceptable 71% load at 12v, expected 380W total load, 76% is ok.
Loses the semi-modular.
Base Total: $927 Mail-in Rebates:-$78.00 Total:$848.91
vs Base Total: $916.91 Mail-in Rebates:-$88.00 Total:$828.91

That changes you from mid-tier to top tier on $20 net, but to get there, had to squeeze it tight.

For AC: Unity, the issue is not the graphics card, though if all goes well,
http://www.game-debate.com/...quirements
GTX 970 @ 248% of recommended vs GTX 760 at 181%
http://www.game-debate.com/...quirements

The issue is the FX-8350 (or 8320OC to an 8350). It is an 8 core design and only Frostbite-3 engine of Battlefield-4 or CRY-3 engine of Crysis-3 are using more than 4 cores. Unless they used a CPU expert in the software design of Unity, it really sees an FX-4350 at best.
http://www.game-debate.com/...quirements
Marginal at game maximum settings.
http://www.tomshardware.com/reviews/gaming-cpu-review-overclock,3106-5.html

It is not FX-8320 vs 8350, but whether 4 cores show idle at all times like most current games.
FX-8320 gets clocked up to an 8350 generally without any issue whatsoever. As shown above, it is the same physical part with a letter marked different and a different AMD final test point. Depending on demand, they could just mark them differently for selling purposes. I worked at IBM+Lenovo and know how the industry works.
Assassin's Creed Black Flag by CPU:
FX series changes only by a couple of frames per second and no hyperthreading. An i3 beats the FX-8350:
http://www.hardwarepal.com/assassins-creed-4-black-flag-benchmark-cpu-gpu/8/
Must play ultra:
http://pcpartpicker.com/p/kr43sY