Well, I'd like to do the bulk of it in plain text so that it gets dated less quickly so ideally it would be text and screenshots.
Video tutorials can also be helpful, but we'd likely post them to youtube and embed on the blog because we'd need a text version anyway for accessibility.
I'd been laying out the help pages, and was using a FAQ format with subsections (e.g. a section on tagging etc). But I can work almost whatever you provide into the layout I need. Preferrably plain text (MS office formatting is a pain to remove) and images for screenshots.
If you do want to do videos (I don't have a good microphone on my computer) CamStudio (http://camstudio.org/) is a good free screen recorder that's easy to use.