Thanks to jim, carregwen, donald and ramblinman for your inputs. Carregwen, I wasn't considering a F/2.8 lens. I am on a tiny budget for now. I thought I ll manage with the 18-55 kit lens for indoors. And go for a 70-300 f/4-5.6 for outdoors, wildlife. from the other threads, I have learnt that there could be AF issues with third party lenses. I don't mind manual focus if it helps to save some bucks. Do I understand right that the Sigma is better built than Tamron and hence a better option?
From Donalds inputs, looks like the idea of Nikkor 55-200mm f/4 may be a better idea. Nikkor 70-300mm/4.5-5.6 is beyond my budget for now. Is there a converter i could use with Nikkor 55-200mm to increase length? Or does it make sense to get 55-200mm now and get the 70-300mm (like Dave did) whenever i have saved for it. The sigma 70-300mm f/4.5-5.6 was tempting at 1/3 price of a Nikkor with equiv specs. Esp since I am likely to be going on a wildlife trip with exotic locations end of this month.
I found a Nikkor 70-300mm at an incredible price at this site. Am I missing something here?
http://www.adorama.com/Als/ProductPage/NK70300AFGB.html
Another reason I was putting off the Nikkor 55-200 for later cos I thought I may decide on a Nikkor 18-200mm if I could afford it later and do away with the 18-55mm kit lens.