Best practices for online help

we're currently developing a fairly complex web portal. To improve the user experience, we want to provide a context-sensitive online help system that can aid the user in understanding certain aspects of the site.
In our case, the site has a variety of widgets that display all kinds of tabular data, graphs, etc. For instance, one such widget may display the VIX and a the help system would offer a brief description of what the VIX is.

Now, I've looked around in the internet and found some interesting articles such as the Design Checklists for Online Help, but most of what I found seems fairly outdated. What I'm specifically interested in are design issues such as these:

• whether (or when) to use popups, divs, or link to external pages
• how comprehensive should the help entry be? how much is the average user willing to read?
• what's a good way to provide access to the help system? cluttering the UI with questionmark-icons is certainly not optimal
• should the help entry be loaded on demand with AJAX (kinda sucks, you want the info right away) or preload it (causing tons of unnecessary traffic)
• other dos and don'ts

The answers to some of these questions may seem obvious, but when it comes to usability I've made the experience that the intuitive answer isn't always the best. Secondly, I'm a software developer and as such I tend to look at things from an engineer's point of view. And I think we all know that this is, more often than not, a pretty poor angle from which to approach the design of a user interface. This is why I would very much like get some feedback from people more experienced in this field.