The normal frequency on my card is 800 MHz on CPU and 1250 MHz on memory. For mining you want the CPU to be as high as possible. Memory on the other hand is hardly used at all, so it should be low to avoid adding unnecessary heat and power consumption. My current speed is 890/500, giving about 324 Mhash. It seems to work at 900 but crashes at 910, so I give it a bit extra headroom.