I am able to get a pretty constant 370-380 Mh/s with my 5870 running the clock speed at 900mhz, the ram at 900mhz and the fan at 100%.  I generally hover around 71 C.  I am just using the CCC AMD Overdrive software.  I am not sure if underclocking the ram really does anything or not. for heat, but I do it anyway.