Yes — a 50mm f/1.8 is a good choice if your goal is shallow depth of field and stronger background blur than your 18-200mm usually gives you. The wide f/1.8 aperture helps both with blur and with low-light shooting, and 50mm is also a strong portrait focal length on DX.

A few things to keep in mind:

• On a Nikon DX body, 50mm gives a field of view similar to about 75mm on full frame, so it behaves more like a short telephoto than a “normal” lens.
• If you want a more normal field of view on DX, a 35mm f/1.8 is often closer to what people think of as a 50mm view.
• The 50mm f/1.8D does not autofocus on Nikon bodies without an in-body focus motor (such as D40/D60-class bodies), so check your camera compatibility.

Also, shallow depth of field depends on more than aperture: move closer to your subject, keep the background farther away, and use longer focal lengths when possible.