Last few months i was trouble with creeping high temp of my sapphire 4850, 64C/100C at idle/load.
Today i took out my gc and observe it carefully.
external part of the card is clean without any dust.
As i look carefully, i realized that there are some dust inside the plastic cover. So what i did is i took the cooler off of my Sapphire 4850, pop out the plastic cover and WOW, lots of dust covering all the fins and the holes where the heatsink connects the fan.
I blew off and vacuum the dust, put everything bc and turn on my pc.
Now my GC run at 50c/75c idle/load. My gc cooled down by 30C!!!!!!!!! wow, holy cow.....
Hope this will help those that having same problem like mine.
The fan will not show much dust but it may be inside the plastic cover. And when dust collects on those fins, there won't be enough air flowing over your GPU.

OMG rm385 for 4870. everyone can own one after this