Was the original jpg at 150 dpi? I am guessing, NO. If the original image was jpg then you don't improve quality in photoshop, you are "smearing" dots. JPG was meant for 72 dpi and 256 color depth. That is about as good you can get since jpg is a math algorithm to fill in missing lines.
You could go to 8 bit color or greys
IMHO, your jpg will make a nice print for most uses.
How about 2 bit color depth = Black and white?