Is this a stock, first revision Radeon 4850? If so, you likely had the same problem I did. Poor thermal grease covering the GPU, and dust in the heatsink.
If you're still under warranty, do this at your own risk, but I'd remove the heatsink from your GPU and clean out the heatsink with compressed air.
Then, I'd clean the thermal grease off the GPU and GPU contact area of the heatsink. Do not clean off the thermal padding used to help cool memory chips and VRM chips.
Take some paste (I used Arctic Cooling MX2) and reapply it on the GPU of the card. If there are any missing areas of thermal padding on the memory or VRM areas, add a very small dot to that area as well. Put the heatsink back on, and you should be good.
I did that and my temps dropped by about 25C both idle and load, with a stock first gen Visiontek 4850.