That's nothing to do with the DPI my friends. That's the required #(screen tones) for publishing materials. For example, normal magazines or brochures require 150# if uses art paper or art card and 133# if uses the simili paper. For newspaper they will ask for 100#. 150# is the standard tone and if you want your printing to be more solid usually, they will ask for 175# or 200#.

For images, standard is 300 dpi. Lowest to go is 250 dpi. When enlarging the image it is recommended to scan or do the image in 400 dpi. Either you make your image big and lower the dpi. It will take the same amount of time to process.

Yes, lower dpi will definitely make your enlarged pic look blur and with jagged edges. If you want to enlarge any images, just maintain the 300dpi resolution, although by enlarging with 300dpi will still make your pic at least 5-10% blur

300-400dpi is the best. No need to venture till the GB sizes. What I usually do is to use the 1 inch : 1 cm scale. Just say your banner size is 120 inch x 48 inch, just do it in 120cm x 48cm.