you left out some details in favor of the story of the symptoms.
Do a clean install and get it to 10.6.8 with your gpu which I assume would be the Apple ATI 5770.
Reset the SMC with hitting power on 10 secs while unplugged from a/c and nothing connected, no peripherals.
Maybe there is a RAM issue. FBDIMMs fail. Almost 90% a given over time due to heat-cold stress.
Monitor the temps closely and the RAM and get the OEM DVD to run its Apple Hardware Test, then do Rember (memtest GUI).
And yes there are threads about high fan speed; yes it is somewhat normal for fans to be high... if the system was overly warm, room is warm, etc. a system cannot cool itself once in sleep or shutdown and heat just sits inside. And the case needs to be where it will cool and fresh air (not in boxed in corner or some "computer hutch").
So i've done some tests and almost certain its the graphics card whose fan is speeding up and slowing down. I've come to this conclusion from the following observations:
1. When I visit a website that is graphically intesive (scrolls through a series of hi-res images every 2-3 seconds), the fan speeds up when switching from one image to the next, but slows down in-between)
2. I wrote a simple python script that cycles through 2 steps. First it does a lot of computations to put a load on the processor for 10-15 seconds, and then pauses for 5 seconds. I performed this cycle for 5 iterations. Under this test, the fan never sped up at all.
Is there some reason why my graphics card would be so stressed just rendering images? If it helps, my video card is the Radeon HD 2600, which is currently driving two monitors at 1680x1050 each.
My EFI driver version is 1.00.252. Is there a newer driver that would help performance?