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.
^ This is the way to go. Also, depending on your card you can move air away from the GPU by placing a fan to pull air out of the side of the case. If you were to place a fan at the base of the card blowing through to the outside of the case and another to suck external to the case would work well. Underclock first though, as adding fans just increases power consumption.