Hi joesphle and welcome to the forum.

Your best method for installing is to use the application manager in the device: that will make sure you get all the bits and pieces of any installation as the application manager will collect them. If data cost is a problem try to do it when you're on wifi.

If you're new to all this, then stick to the nokia catalogue and the Maemo-extras catalogue for now. The Ovi store is not actually live, though it's been visible briefly in testing. Avoid the 'extras-testing' and 'extras-devel' catalogues as the apps there are still being created and might not do your n900 good. (Until you're happy to tinker a good bit more).

Good luck hunting. Maemo-extras currently has 70 downloads. You can also lok at a large collection of wallpapers at the n900 wallpapers site.