Thank you all for the replies. DrMrLordX, I am very interested in this, in addition to adding high cfm fans. Would this be like undervolting the card? Do you have a link to show me how to do this? Thanks!
Yes, undervolting is the easiest way to go about reducing power usage. Many people use MSI Afterburner, but I have switched to Sapphire TRIXX since it allows one to reduce voltage by as much as -200 mV instead of -100 mV (which is the limit for Afterburner).
The first thing I recommend is getting the software here:
http://www.sapphiretech.com/catapage_tech.asp?cataid=291&lang=eng
Next I recommend using the "custom" fan profile for your 390s (the one by default is more aggressive than what is set in your card's BIOS). After that, I highly recommend you install and use GPU-z:
https://www.techpowerup.com/downloads/SysInfo/GPU-Z/
It will show you power usage as VDDC Power In. On my system, it is always 75W too low. Anyway, try to get your power to 200W or lower (should show ~125W in VDDC Power In). A good starting point is to lower the card's clockspeed to 900 MHz and then lower voltage until it starts to crash the driver (bear in mind that the voltage setting you pick will always correct +7-8 mV thanks to some odd behavior of TRIXX). Monitor power usage. You should be able to get your power usage to around 75-80W according to GPU-z, which translates to ~150W power draw from your PSU.
I realize that moving GPU clocks to 900 MHz is a big reduction in performance (from maybe 30 MH/s to 25), but your GPUs are running far too hot, and you need to get heat down on both of them for them to survive.
Please note there's no proposal for a hard for (yet). I was confused about this as well (see my post below) but YOU NEED TO VOTE FOR A SOFT FORK first to give time for the developers to create the proposal for a hard fork.
https://www.reddit.com/r/ethtrader/comments/4pe64s/why_are_mining_pools_limiting_miners_to_soft/
Now it makes sense. Thanks!
What OS do most people recommend for Ethereum mining? I was planning on Unbuntu...would this work well? Thanks.
If you are mining with an older card (Hawaii or Pitcairn) you need to use a Linux version that still supports the old Catalyst 15.12 drivers. The open-source radeon drivers do not support multiple GPUs to the best of my knowledge. One of the easiest is Ubuntu 14.04 LTS, though for a small footprint I recommend the Lubuntu variant.
You can use Lubuntu 15.10 if you want newer kernel features, though be warned that support for that OS (security updates, maybe not that big of a deal) will end well before it end for 14.04 LTS. LTS = long term support.
If you mine with something newer, like Fiji or the upcoming Polaris cards, then you can go all the way to 16.04 since you will be using the amdgpu driver, which might support all the features that you want. In fact, for Polaris, that will be your only option, since Polaris will be unsupported by the old Catalyst drivers.
I'm not 100% sure that mining with amdgpu will work exactly as intended, but I'm at least somewhat certain that it will. amdgpu doesn't yet support hawaii/pitcairn so that is not an option.