I won't type alot, because I do not like to, but here is a quick way to greatly increase the quality of black and white conversion. First duplicate the layer, and image/adjustments/desaturate one of them. Now hide it.
Take the other layer and go into the channels. Usually, any grain or dust from scanned photos, or grain in digital photos will be on this layer. Look for which layer produe the best contrast and bring out what you like in the picture. Now go back to layers. Click on the layer(can't be a background layer and in RGB) then click image/adjustment/channel mixer. Click monochrome in the bottom left corner of the prompt box. Now adjust away, using your preferences from the different styles brough out by the different color layers (now each one is B&W) The three colors together must equal 100. If you go above of below 100, use the "constant" in the bottom to compensate.
Remember, if you desaturate, and then use contrast adjustments and like to achieve the same picture I made, you will be in escense losing quality. By putting the right amount of each of the RGB channels after monochroming them, you will get the effect without losing any information or adding noise.
Enjoy!
Brendan