Thanks, Zig. I have quite a wide range of photo interests, from wild life, to fireworks, to landscape, to flowers, to sports and events. Probably the least interest is in portraiture and studio type stuff, for some odd reason, although I do enjoy natural people shots. I have held off on adding any more lenses until I am sure I am committed to Nikon. I was also considering the Canon 60D. But, I am a sharpness freak and do aspire to have the best glass I can afford, once I upgrade the body. (the lens limitations for the D40 being a main driver for the upgrade...)
I'll have plenty of time to learn the new body while I wait for my finances to recover before buying a new lens. It's nice to know that my Tamron won't be a drag on performance.
My flickr site has some samples of the subjects I enjoy if your are interested in a look. I really feel that my D40 has served me well. The cheetahs at Busch Gardens here in Tampa are my latest favorite subjects. Their new habitat is just wonderful and they are so photogenic! http://www.flickr.com/photos/tropidale/
Thanks again for the comments.