I installed update-manager on my Debian Lenny today. This installed an update-notifier daemon. I noticed a sudden jump in ram usage.
Top reports of a shared-memory-usage of 8.5 MB,and a total of 11 MB. For a new computer with sufficient resources, this is acceptable. Also on my current laptop ( Dell Latitude 1.8GHz 1MB ram ) this is acceptable on my Debian Lenny install, given that my memory usage is still below 50 MB when no other applications are running besides Conky and Update-notifier-daemon.
The only real gain using the ubuntu or debian update-notifier, is that you get notified whenever there is a repository update on installed packages on your system. Also, the daemon is generally shown in the system-tray whenever an apt-get or update-manager is running (it does so on Hardy Heron, but no so much on my Debian Lenny – go figure…).
On old computers or systems where you wish to save booting time and resources, specifically ram-usage, I recommend running apt-get update as a cron-job or manually, rather than installing the over-resource-hungry update-notifier/update-manager. Synaptic does the same job manually together with apt-get. Notice that you can simply disable the update-notifier-daemon.
When killing update-manager, and checking memory usage with free before and after, I only free up roughly 3 MB of memory. Not that much. However, I suspect the memory usage will go further down if I remove the installed daemon completely.