Thanks for the answers!
Another question: I heard that you get more hashes/s underclocking the GPU memory to 300MHz,
From CCC it seems that I already have it at 300MHz
What could I do to improve the performance? I can't overclock since I already get too high temps.
It's at 300 because it's not under heavy load. It will rise just as you try to run the miner.
You won't get higher hashrate with lower memory clock, but you can decrease power consumption and heating a little bit.
~40W less on 5970, for example