I can get by with 50 % fan speed at 5870 speeds (850/1200) on my 5970. Runs fine, tested under extended sessions of Crysis. Honestly, overclocking it is pointless ATM unless you're benching. In most games I average well over 100 fps at 2560x1600, an extra 10 fps there means nothing and isn't worth the extra heat / power consumption.
One thing I noticed, at least with my 5970, is that it requires much less voltage to reach 5870 speeds. I can get a stable overclock of 850/1200 at only 1.125 Vcore. That's significantly less than the stock Vcore of a single 5870. I assume with proper cooling these things could be pushed quite far..