Currently running at slightly below 70C at 70% fan speed and the case open.
How did you get your core to 900 Mhz in Ubuntu? I can't go above 775 Mhz on my core with aticonfig. Are you using a custom program?
In Ubuntu when I run
DISPLAY=:0 sudo aticonfig --odgc --adapter=all
I get this:
Adapter 0 - ATI Radeon HD 5800 Series
Core (MHz) Memory (MHz)
Current Clocks : 790 1000
Current Peak : 790 1000
Configurable Peak Range : [765-918] [1000-1200]
GPU load : 99%
Which means you can change the core clock in the 765-918 range and the memory clock in the 1000-1200 range.
So to change the core clock for instance to 800 Mhz:
DISPLAY=:0 sudo aticonfig --odsc=800,1000