I've noticed a tendency over the years for people to use "geek" to refer to people who know a field especially well -- and it doesn't have to be a technical/scientific field, either; you can be a computer geek or a math geek, sure, but there are also history geeks, literature geeks, political science geeks (the so-called "policy wonks" you heard so much about in the early days of the Obama administration are clearly poli-sci geeks), and many other types. (The
Geek Code has been around since 1993, and includes geeks of all flavors.)
Whereas "nerd" seems to be applied more, generally, to people who are socially awkward and have trouble making friends. Some nerds are also geeks, but many aren't, and many geeks are not nerds. The ones who are the most socially awkward are sometimes diagnosed with Aspergers, which is somewhere between "normal" and "autistic" on the autism spectrum. Most nerds, though, may have some autistic tendencies but tend to fall closer to the "normal" end of the spectrum.
I think there's some value in having different words to describe different types of personalities, as they can reduce confusion and ambiguity in discussions like this. In particular, I think Dale and Marigold could be descibed as both geeks (gaming geeks and manga/anime geeks)
and as nerds, with Dale being slightly less nerdy than Marigold.