Fred,
Its impossible to recommend a lens for somebody else, what I may find acceptable, you may not. Having said that, I would have thought the 18mm short end of your current zoom is wide enough (but as I mentioned in your WA lens thread, I hate anything wider than 50mm anyway).
For landscapes you don't need a particularly fast lens. i.e. one that has a large maximum aperture, usually f1.2 to f2.8. So no need to spend the extra on those types of lenses. These are only really needed for low light/action photography or for special effects like background blurring (bokeh).
With the crop factor of your camera (sensor size relative to a 35mm film frame, so multiply any focal length by 1.5 to get the equivalent field of view on a film or full frame camera), its hard to find a truly wide angle lens. I think 10mm is the widest you can go, thats on a Sigma lens, I don't know but but I think Tamron and Tokina both do one that starts at 11mm and I've no idea what Sony does.
I also think (remember, I don't really know, because I don't shoot any wide angle stuff), that once you get lower than 24mm you start to introduce perspective distortion into your pictures - Though you might want that for artistic effect.
Hope That Helps (a bit)
