Lynn:

The 10-24 on the D90 (or any DX camera) is indeed equivalent to 15-36 on an FX or 35mm film camera, just as your 18-105 is equivalent to 24-160 (or thereabouts). So you're either getting "8mm" in DX terms or "12mm" in 35mm/FX terms of extra wide-ness.

For what it's worth, I have the Tokina 11-16 for a wide lens, complementing my 18-200. My third lens is a 35mm/1.8, which I found very useful last year for the night shots - despite the relatively narrow field, the extra speed shortened the exposures by a significant time. If you happen to have access to someone's 50mm lens from their film days, that would work just fine as well.

Scott