1) You do not need an unlocked CPU like 3570k. Unless, you want to overclock the cpu and, then, you need.

2) No need. GPU OC is GPU, CPU OC is CPU. They are completely different and separated.

3) In MSI afterburner, with stock voltage, try to increase the core clock by 10/25MHZ and run the test (as stated in no.4 and u might no need to play the game for so long, lets say 1 hour enough), if it passes you can increase another 25MHZ until the test fails and you lower back to the previous clock. After the core clock is stable, lets move to memory clock, increase 50/100MHZ once and do the stress test like previous ones.

4) It depends on the gpu chips. Every card has different oc capability. Remember to monitor the gpu temperature during stress test.

5) Since you are not overclocking a cpu, so no need to check that. Temperature do not exceed 80 degree celcius during load testing (100% usage of gpu). To test for GPU stability, try run the benchmark of "Unigine Heaven 4.0" for three times at least, notice any anomalies or artifacts during the test. And play heavy directx-11 games like crysis 3, battlefield 3 for two hours minimum, to see if anything strange or Blue Screen of Death happens. If there is nothing, and I can say that your overclocked gpu is stable for 24/7 usage.

6) Nope, you just want to overclock the graphics card. So, Sapphire 7870 XT is a real decent card and has good overclocking capability.