New Graphics Card Not (Properly) Detected

Hi,

So my main install / system is Mountain Lion 10.8.5 on an HP z800 with Dual Xeon x5670 6-core processors @2.93GHz
48GB ECC Registered DDR3 1333 RAM installed (12 x 4GB sticks).

In an attempt to improve my system, I swapped out the old / underwhelming nVidia Quadro FX380 that I had been using for a Sapphire Dual-X R9 270x card...

Now, after a few boot issues and two audio-related kernel panics (which I'm guessing must be somehow related to the HDMI port's potential for audio output?) I...

New Graphics Card Not (Properly) Detected