ATI Radeon HD4850 running hot

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.

Should I worry? Using a stock cooler fan

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.

What case do you have? 4850 don't blow the hot air out. Your case need to blow hot air out and force cool air from outside.

At idle or none 3D apps the temperature should be way below what you are getting.

How long did you have this card? Have you check for collected dust on the heat-sink that blocks the air flow?

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).

Mine idles at 40-45C, 60-65C at load, automatic fan. Its dual slot though.