This dropped my temps from 89/92 to 77/74 degrees centigrade thats with the cores at 900 and ram at 900.
You should underclock the memory. My 4870 went down 10 degrees by lowering from 900 to 190.
And if you hit the sweet spot, you'll gain a few MH/s. For me it's at 190, for 5xxx I've read 1/3 of core.