What is the safe operating temp for the HD 6870s?

85 degrees is safe for full load, if a bit hot. GPUs are designed to take higher temperatures, as long as your temps don't creep up into the mid 90s or higher you should be okay. However, GPUs will last longer if you do keep their operating temperatures lower. You could use MSI Afterburner to set a more aggressive fan profile. The default fan settings on most cards tend to be inadequate if you push the GPU to full load for extended periods, as manufacturers tend to focus more on making their cards quiet rather than ensuring the best cooling. Setting the fans to speed up to 100% at a lower temperature (say 70-75 degrees or so) may help, by default the fans usually won't speed up to 100% until the GPU temperature hits the mid 90s.

Thanks for the helpful reply! A couple quick follow-up questions:

1) Will that MSI afterburner program work with the XFX branded version of the card instead of the MSI version?

2) Will it conflict with the ATI Catalyst Control Center fan control?

3) Where are you getting the information about safe GPU temperatures from? I couldn't find anything from AMD.

Thanks again!

yes the afterburner will work with any brand of gpu.

no it will not conflict with the CCC fan control

and yes he is correct. 85C is a bit hot, but still ok. 90's is where you will start to have trouble.