I had a major power throttling experience on my 7800 GT (Flashed OEM ROM v2152.2) in my Quad G5 as documented
here, which was resulting in a consistently low OpenMark score of 1254, under both Leopard or Tiger.
I tried the display sleep technique above and I couldn't hear (or see) the fan spin up as reported by others, but I did finally resolve the issue by changing the performance profile in System Preferences. The GPU now benchmarks exactly 9331 as reported by many others.
Something had locked the GPU state in its low performance mode, which was not addressed by PRAM or NVRAM resets, so I just wanted to share, that if anyone is seeing lower than expected benchmark results, then try using Energy Saver to toggle between Automatic/Highest/Reduced and see if this makes a difference.
I am able to see the single sphere render window jump from ~750fps to over 1400fps when I switch between Reduced performance mode and Automatic or Highest (while in it's throttled state, this was sub 100fps).