Go to settings in MSI Afterburner,
at
Compability Properties tick all settings,
Under Enable low-Level hardware access interface select
kernel mode
And at
Unlock voltage control select
Third Party
At bottom
AMD Compability properties
Tick
Extend official overclocking limits (Bear in mind that you cannot OC that much.
Tick also
Reset display...
Go to AMD control panel, not sure which tab since im on Nvidia side, reset default the AMD control panel soo you dont make
Fighting between AMD and MSI Afterburner.
Safe voltage for 7870 is 1.25V~
Safe recommended temp under 85*C~
Before doing any overclock at all, run a default Run in unigine valley or similar benchmark like furmark to see your baseline temperatures, for instance running an Furmark for around 1H to see your max temp, for instance mine would be 65*C without touching any fan curve or anything else.
After overclocking (since i cannot control voltage) is same since the cooler is way overpowered for this GPU.
After doing an overclock you do an stability test on Unigine Valley (one run for benchmark) and see if it crashes, if it doesnt you keep doing overclock until you hit wall (usually drivers crash of freezes the program).
Then you revert back the overclock for 25MHz and do again.
After you established max overclock for core with voltage bump (this is if you are willing to do) then you go for memory, same goes for stability test, overclock as far as you can without freezing PC, then you revert back memory clock a bit.
Hope this helps, this is from my experience.
IF you need video tutorial, here it is ->
https://www.youtube.com/watch?v=vUEMS-B1Siw&t=57s