Being a pedant... it is ppi not dpi, the latter refers to the printed image i.e. dots per inch laid down by the printer on the paper.
Displayed on screen then the ppi makes no difference it is the pixel dimensions that count, i.e. an 800px x 600px image at 10ppi will display the same size as an 800px x 600px image at 100ppi on screen.
The relationship between ppi and pixel dimensions will and does make a difference when going to print and the final (native) size of the print, i.e using the same example as above the 100ppi image will print smaller than the 10ppi image even though the pixel dimensions are the same.
So by reducing the ppi you can print larger whilst retaining the pixel dimensions, the larger the print the greater the viewing distance will be so the lower ppi will have no real effect.
Most commercial printers will suggest a ppi setting for your files, their software will do a good job of resizing the file for various sizes, 300ppi is always a good starting point.
As others have stated experiment a little.