Is it possible to convert a system app to a user app and then uninstall it or move it to SD?
I'm thinking about not critical apps, such as Astro filemanager (I use Root Explorer), Google maps, Google Talk and so on.
The easiest way to do this is probably to use Titanium Backup. If you long press on a system app in the list it will give you the option to "Convert to user app". Conversely, you can long press on a user app and "Convert to system app" as well. Note, though, that some system apps will not function well as user apps because their permissions may be restricted. Most of the apps you seem to be considering specifically will probably not have issues, I would imagine, though Google Talk actually might since it's somewhat tied into the Market (not sure).
Further, Titanium will allow you to directly uninstall a system app if you want to, without having to first convert it to a user app. All of the above requires that you have rooted your phone.