I already know that these cards run hot but I'm running 81'C by default and fan speeds at around 24% using Catalyst fan control. If I put fan speeds to 50% it will idle at around 66'C. I can easily hit the 100'C mark when playing heavy games such as Left4Dead. It has never really been unstable but the heat is kindof worrying me.
I have two fans blowing out air and my whole chassi is pretty much open with small holes covering the whole top and several bigger holes on the side to allow air to come in.
I would buy an 80 or 120 mm fan and put it in a way so that it feeds cold air into the stock HSF. You could also buy some Arctic Cooling MX-2 for 7$ from newegg and apply it to the stock HSF instead of the stock thermal compound.
That is what I did, and my Sapphire HD4850 idles @ 30C and the max i've seen is 55C.
This is normal, my 4850's run at the same settings if I leave the fans on auto. I manually adjusted the fans and setup profiles. "Quiet Desktop" keeps the fans at 35% for one of my cards and 40% for the other. This usually keeps me in the high 60's to low 70's. When playing a game I have the fans set at 65% and 50% (65 for both if crossfiring). This keeps my temps in the 70's - 80's. The cards are designed to be able to handle heat so ATI lets them run warm.
Is this one of the original (single slot) cards? My 4850 will push 90-100C in certain games (even though it idles at 40C). Check and make sure it is clean and turn the fan to around 50%, and that is all you can really do for it beyond replacing the cooler (which I really should get around to sometime on mine).
If you do not want to hear the noise of the card's fan, and you have 10$ to put to use, buy 2 80mm fans and install them in a way to feed cold air into the card (one horizontally and one vertically). That will be a much quieter and easier solution.