Not really sure about the internal hub gears like the sturmey-archer that come on the Pashleys. The details I found at this site talk about Ratios, but I'm not sure how that compares to your Marin or Spec mountain bike with standard cassette in the back. It's hard to know the top end of the gear ratio, ie how much mechanical assistance your legs are getting, but the 3 and 5 speed bike here are mostly for the flats and I'd think you would struggle up any sort of incline or pulling.

http://www.sturmey-archer.com/products/hubs/cid/4

Globe, made by Specialized I believe, and the local shop here sells them and I have to tell you I was NOT impressed with the build quality. Friend who I had my little bike incident with a few weeks ago owns a Globe and when we crashed at relatively slow speed (10-12mph) his wheel was totally ruined. Asked the wrench about the wheel and he explained that the stock wheel does not have spokes with crossed laced pattern and makes a wheel weaker, but easier to make. My bike was unscathed and functioned great. Of course my bike cost about 2x what that Globe did too!

Shannon