Decided to bring this over from the Bethesda forums where I originally posted it.

NOTE: The fan fix more widely known of for the Radeon HD 4850 is not an adequate solution to the issue hereby addressed. This issue is one where the basic plastic cover on most single-slot models of the 4850 actually serves to trap dust against the heat sink until all cooling value is lost and the card begins to overheat drastically to the point of damaging the card itself.

NOTE: The following is not advisable if you don't know what you're doing or if you have what is more colloquially known as "butterfingers". This is not advisable if you value any manufacturer's warranty you may have, and I hereby take no responsibility for you destroying your expensive graphics card by mucking this up.

***THIS TECHNICALLY VOIDS YOUR WARRANTY***
***THIS IS NOT TO BE ATTEMPTED BY THE INEXPERIENCED***
***DO NOT CRY IF YOU SCREW UP***
***KEEP ALL PARTS SAFELY STORED TO REASSEMBLE IF SOMETHING GOES WRONG, AND PRAY YOU CAN GET BY THE MANUFACTURER***

The easiest way to fix a stock single-slot 4850: Remove the plastic cover.
The plastic cover on the 4850 is nice and pretty and all, but it serves as a trap for large amounts of dust which accumulates in a short period of time. After an alcoholic beverage, I became bold enough to say to hell with things and decided to dissect my videocard. Removing the heatsink by (WITH POWER OFF) unplugging the card fan and unscrewing all the screws on the bottom of the card with a small NON-MAGNETIC screwdriver. With the heat sink in hand, I noticed that the plastic cover was easily attached with a separate set of small screws. Taking these out, some of which were hidden just under the corners of the cushioning tabs on the heat sink, and storing them safely in a small pill bottle, I removed the plastic cover. With it removed, I noticed it had caused a vast quantity of dust to accumulate against the heat sink, and screwing up the airflow. I cleaned it out, CAREFULLY aligned it back with the card, and screwed the heatsink back onto the card. Reconnecting the power to the card, I plugged my PSU back in and hit the switch. Since removing the cover, I went from ~75*C at 100% fan speed to ~50* at 50% fan speed.

If you attempt to try this, good luck, and be steady. Hope this helps you. :)