If you are doing landscape photography then you should really be using a tripod. This means that low light performance of a lens (as you mentioned with you 18-200) is not an issue, as you can use as slow a shutter speed as your require to get a crisp sharp shot at F16 (or whatever you are using).
You also seem to be focusing on the "specs". We all tend to get carried away with the specs at times but good photography is possible with basic equipment. As this is also especially true for lenses.
I read a quote (sorry can't remember who) which said something like "I have seen fantastic photos taken with an iPhone and terrible photos taken with a top of the line camera. So it really depends on the photographer and their ability more than the equipment."
Although a higher price lens might have better specs, the quality actual comes from the fact that better components, glass and manufacturing processes were used (which cost more).
It is better to try and find some independent reviews online that compare lenses in your price category to see what the real world delivers from them. You will often find that a lens which has worse specs has better quality.
You will often find that most lens has a compromise somewhere, it is just what is acceptable a compromise for you.
As for a lens, one suggestion is the "Sigma 10-20mm f3.5 EX DC HSM". I have this (for my Canon) and I know others who have it and use it for landscape and we are all happy with it. The price is reasonable and the performance and quality are good for the price, although you should stay away from the extremes of the zoom if possible as the performance drops of here.
But I see no reason why you can't use your existing lenses for landscape photography. Just go out and practice! Practice makes a photographer better, not a new lens.
