I used to have a 6870 GPU and there isnt much overclocking you can do. Even with voltage adjustments and proper cooling I dont think you will get very good results. I had my 6870 core at 1100Mhz (yes a bit extreme) and it yeilded very little improvement in game performance (arround +5FPS at most in BF3 High). For this reason I just ran at stock clocks.
I know a few other people that I game with that have owned 6870 cards and they had similar experience. I think most of this is due to the locked TDP of these cards. After upgrading to a 7970 3GB which have adjustable TDP to +20%, it makes overclocking much easier with fewer problems like the ones you are experiencing.
As for your temps, that doesnt supprise me. While 85C is okay
, it's not ideal. I would recomend finding something you can keep under 75C without having to put up with a fan that sounds like a hair dryer. Also keep in mind that your Vram temps are usually not shown in temp monitoring programs and they could overheat.
Hope this helps you out and good luck.