Visually differentiate external links?

I'm in the process of redesigning this one website that has a good mix of internal and external links in the body of just about every page. A lot of the visitors are middle-age and many have asked to make external links open in a new window automatically. So i was thinking of making the external links look slightly different from internal ones, to give the visitor a clue of where they're going. Maybe use a dashed underline instead of a solid one, coupled with "External website: " in the link's title attribute.
Any thoughts on this? Any usability studies on this subject? Or is there a better way? Thanks. 


I like Wikipedia's approach. Small image for external links, apply the "External Link" title to it.
Then again, it is actually much easier for those relying on screen readers then it is for everyone else. Screen readers will read the URI. It's the seeing that need the most help, in which case a clear visual cue is best. 


My personal preference would be to just use a tooltip.