After having overheating problem with my Graphics card Sapphire ATI Radeon 4890 1 GB (GPU Clock 850 and Memory clock 975 MHz), I am thinking to reduce its GPU Clock. I found different ATI's 4800 series's GPU and memory clock:
My current fan speed is 50%. I am using Full tower chasis Tharmaltake Xaser VI. I have not recognized my room temp. properly yet. But my room gets too hot during summer season. Previously, I owned a low tower chasis which caused overheating issue constantly. So I replaced it with Tharmaltake Full tower.
Now my system gets frozen if I use it more than 6 hours.
From BIOS, CPU Temp. 30 to 33* C and Overall temp is 40 to 41* C. From Vista 64 BIT sidebar CPU Temp. is 33 to 40*C
You can Google for ATI Tray Tools which you can use to underclock your HD 4890. I would lower both Core and Memory speed. You can also lower voltages, but it is a little tricky and should only be used as a final option.
The higher your room temp, the less effective a cooling solution will become. If indeed it does get hot in your room then underclocking is a smart thing to do.