us18x, each model GPU has different characteristics, and the memory used limits speed. There haven't been a lot of reports yet on OC'ing the 870 so I don't have a good feel what the typical limits are.
The general rule on an unknown new chip is to start slow, and watch the temperatures - start with the GPU first, at say a 10% OC - then increase by a reasonable increment until you start to get rendering errors - sparklies or row tearing or color weirdness - then back down half of the last increment and test again - etc. Then run a number of benchmarks and stress tests - constantly watching the temperature - and see if anything weird happens - like display driver stopped... once you are comfortable with the GPU OC, then start on the memory OC.
Same thing with the memory OC. Start at 10%, find a good increment for increase, and test the same. You can keep the stable GPU OC while finding the crush depth for the memory speed - watch for graphics anomalies - and when you get them the first time back down 1/2 increment and test again.
When you are sure you have stable numbers, create a new thread to post them!
Have fun! 🙂