For Q#1, see the wikis on both this site and on maemo.org.
One Quick answer - use a program called media converter to take any AVI and make it the right size and framerate, etc. to run on N810. Its very fast and easy (but there are other programs too).

also, search the forum for "video convert" to find some good links - maybe do an advanced search to look only in newbie or another forum.
enjoy (I only have a 770!)

I think there might not be any great PIM software, but not sure, others can tell what they use. or look around on maemo.org