I don't push them too hard (1000MHz core and mem clocks) and get around 560-580 kH/s with temps of 65-70 oC. If you're willing to push the core clock higher than this then I believe that you can go higher. In my hands the mem clock has less of an impact (even for scrypt!) In terms of temperature control, there's a big difference between one card and multiple cards, so think hard about your cooling solution. You'll need to deliver lots of cool air to the GPU fans to avoid them being heated up by their neighbours. Good luck and enjoy!