JoJo, did you by chance get a book called Bernina accessories when you got your machine? I got it with my 730. I use it all the time to see what each foot is for.
The 34 says it is ensures a good view of the stitching and needle area and it is used for forward & reverse utility & decorative stitching. The C has something to do with the sensor eye. Does your 640 have that? My 730 does.
The 37 is the patchwork foot and is for any machine. That is what I use when I am piecing. I also have a 57 foot for piecing that has a guide on the side, but really bought it for when I'm teaching my granddaughters how to quilt. I always use the straight stitch plate when I am piecing on my 730, because it is a 9mm machine and the opening is bigger. When I piece on my 170QE, I just leave the regular stitch plate on and don't have any problem.
I didn't put my walking foot on, but I have to tell you that I have used it many times and have never put the that side guide thing on it and it works just fine. I usually use the walking foot when I sew on binding because the layers are so thick and also sometimes use it for quilting if I'm doing something like stitch in the ditch.
I'm not sure about your bobbin question. I did buy a separate bobbin for bobbin work and it looks just like my regular bobbin. My embroidery machine bobbin has a little spring in it. But that could be different on the 730 than on the 640.
I really want to buy the Bernina Feetures books, but they are sooooo expensive. Maybe someday.
Hope this helps. Sherry