With the radeon open source drivers the power management uses the 'default' power profile for the graphics card, which many manufacturers set to to be the same as the high performance settings (so you get all the heat and the fan spins at high speed).
See here for an overview:
Try out this command to see if it cools things down:
$ su -c 'echo low > /sys/class/drm/card0/device/power_profile'
If you want to set the profile at every boot then you will need to create or modify /etc/rc.d/rc.local - here is the one that I use:
# Set radeon power profile
echo profile > /sys/class/drm/card0/device/power_method
echo low > /sys/class/drm/card0/device/power_profile
Also make the file executeable:
# chmod +x /etc/rc.d/rc.local