Tanya,
Sorry I haven't checked this thread in a while.
From New Ulm, I would suggest also going to Pipestone, MN. It is not far, and quite interesting. From there, you can easily get to DeSmet. From DeSmet to Fargo, I really would suggest taking the interstate. There really isn't anything special along the way that would warrant not taking the interstate. I second the suggestion for the Terry Redlin museum in Watertown.
http://www.redlinart.com/
You mentioned US 25, but I don’t think this goes through SD up to ND. Again, I really would take the interstate. The view really isn’t going to be much different on a side road than on the interstate, and it will be much faster on the interstate.
As for Bemidji, I would suggest doing Itasca State Park (where the headwaters are) on your way from Fargo to Bemidji. Itasca is really a wonderful park, and worthy of at least a one night stay. I would stay there rather than in Bemidji.
Bemidji is an ok town. Make sure to take the obligatory picture with the Paul Bunyan statue on the shores of the lake.
Where are you thinking of going from Bemidji?
I’ll check this thread more regularly in case you have any other questions.
Josh