I'd say the 1em's have it right. If you're really concerned, provide an obvious in-page style-sheet switcher that bumps up the font-size. Wired has a good example of this.
I'd say there's a distinct difference between marketing copy, and article copy. For good examples, contrast Amazon's styles with the New York Times website's styles.
font-family:georgia,"times new roman",times,serif;
Which of those would you say is more readable? I think they both work well for their intended purposes, but if I'm going to be reading for a bit, I want serifs.