Re: ATI HD4850 heat problem
The best way would be to one or two fans directly on the passive cooler to install. A fan in the side panel could also bring something. Yes, then the card is no longer a purely passive cooling, but 80 or 120 fans are definitely quieter than the original.
The open enclosure is in your case quite well, but for a proper airflow in the cabinet rather than help.
Re: ATI HD4850 heat problem
To exchange the card, eg for a Gainward 4850 Golden Sample, which has a good cooling system, the fan is installed, manually set, very quietly.