i had this problem of huge fan noise when i'm using fglrx driver for my x1950pro card. I recently found this:
It's basically a modified radeontool that tweaks some registers in the card to turn off unused rendering pipes.
I tried it myself and it works fine - card quietted down after 15 secs after turning it on, so i'm putting the link here for people in need.
It seems to work fine with newer generation of cards, like x1600+; there have been reported hard lockups when using it on a x1400 radeon. No reports for the effect on the 2000+ series yet.
tar xvf *tool.tar
cp radeontool /bin/radeontool_power_low
radeontool_power_low power low
i atattched the modified radeon tool in case the original web site is down for some reason
if you get some error like: "Radeon control memory not found" try the attatched version