I'm using aticonfig to clock a HD5870 to 1030/300. With two external, crudely rigged, 120mm fans pushing air in the general direction of the graphics card and the gpu fan at 75% the temperature is 65-69ºC.
Here are some commands.
aticonfig --od-enable #enables overclocking
aticonfig --pplib-cmd "set fanspeed 0 x" #replace x with the percentage you want the gpu fan to be running at, auto for auto
aticonfig --odsc=x,y #replace x with clock freq and y with mem freq
aticonfig --odgc #shows current clock speeds
aticonfig --odgt #shows current temperature
There are lots of other commands available (aticonfig --help) but those are the ones I find most useful.