If you run Debian Lenny Synaptic will not help you to get your HD5850 running, the drivers in Lenny are simply to old for this card. There are two possible solutions to get your card working:
- Upgrade to Squeeze and then install the drivers from the repository.
- Install the drivers from AMD's website.
I personally would recommend the first way, because Squeeze soon becomes the new Stable, and because you will get automatic updates for the drivers. If you install the drivers from the website, you have to reinstall them every time you get a kernel-update, or your X will not longer work.