DPI is just a set of instructions to be sent to a printer. Where you print 72dpi, it will print 72 dots in one inch. At 300, it will print 300 dots in one inch. This has no measure on the actual number of pixels your image contains.
Viewing on screen, it makes no difference. If it bothers you, simply go to PS and adjust from 72 to 300, but be sure to uncheck Resample Image.