Depending on how long you want to use your card, 1GHz might be not that good of an idea.
I have 5x5870. They are all running 412MH/s at 950MHz core clock and 220MHz memory clock. None of them exceed 75C temp. You can push it further to 420MH/s but it just consumes more electricity with no real gain in processing power
I'm getting 413 MH/s on each of 2x5870 at 935 MHz core and 319 MHz memory clock, fans at 100%. The lower card runs at 45C and the upper - at 75C (cards are too close - bad mobo).