WYSIWYG export

You can press the easy button on NA1400's slippy-map Garmin generator or Lambertus's premade images.

Unfortunately I think that the simple answer is that it's not straightforward.

The way the map looks on the main OSM site is determined by the software running there (and there's a choice of at least three maps on there - the "+" sign at the right-hand side that allows you to select "Mapnik", "Osmarender" and "Cycle Map").

The way the map looks on a Garmin is determined by a couple of things - the style folder within mkgmap (which you can tinker around with a bit - that maps OSM items to Garmin items with predefined Garmin bitmaps etc.) and the TYP file (that allows you to tinker some more and not just use built-in Garmin items for OSM items).

I've never seen a Dakota but from looking at the pictures on Garmin's web site it does look like its a bit more capable colour-wise than the eTrex that I have. You may be able to make the map look a bit more like the main ("Mapnik") OSM map by creating tinkering around, and if you're doing this the main mkgmap mailing list and archives might be helpful: http://www.mkgmap.org.uk/pipermail/mkgmap-dev/

If you haven't already seen it, there are a bunch of links here: http://wiki.openstreetmap.org/wiki/Mkgmap

Also, a quick web search for "TYP file" will point to people other than OSM folks who are doing much the same thing.

There is a TYP file here, that is a similar style to the Mapnik map on openstreetmap.org: Mapnik layout (TYP file) for Garmin osm maps

You could try that on your Garmin and see what it looks like. Though you are not going to get it looking exactly the same as the map on the webpage.