It is a good idea to use the 18-200 for a while to see what range you tend to shoot at, then you can buy another lens to use once you know what your needs are. I did that with the D60 and an 18-55 and 55-200 lens.
The 18-200 is a good walk-around lens, and as a pp said, will give you plenty of range so you can narrow down your future lens needs.
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Nikon D90: Primes: Nikkor 35mm AF-S, Nikkor 85mm, Nikkor 105mm VR, Nikkor 180mm. Zooms: Sigma 10-20mm, Nikkor 18-135mm, Nikkor 18-200mm VR. Point and Shoot: Panasonic Lumix Tz5.