D5000 upgrade lenses

Hi all,

I have a D5000 and have been using my 18-55 lens. I am just a hobby photographer and some of the photos which I have taken are available at
http://www.flickr.com/photos/92952410@N00/

Most of the times I take close up shots or landscapes for which my current lens is fine. But sometimes when I go to the zoo or to places where I need a bit of zoom, I find this lens is not sufficient. Also when I am in zoo or any other places, I would like to take close up shots as well and hence not willing to change my lenses often. I cannot spend lots on lenses as well.

With these constraints, I started looking out. These were the lenses which I found
18-200 nikkor
18-270 tamron

Apart from these I also read about 55-200/70-200 also. I was also interested in the 50mm.

Now I am basically confused as to do I actually need additional lenses?

Can anyone suggest?

Thanks for your advice and help.
Shankar.