I wouldn't normally want to direct you away from this nice forum, but you should ask this at a forum that deals with photography in general or preferably Nikons specifically.
It's like how long is a piece of string. What's your budget? Do you crave critical sharpness? hate CA? Need very fast and accurate AF? Hate any f-number slower than f/2.8? Nikon only or 3rd party? What do you shoot? Birds (feathers on)? Landscapes? Street candids? Or everything?
I can tell you one thing: with a D60 and Nikon's strange "will we, won't we" approach to moving the AF motor out of the bodies and into the lenses (should have done it years ago if at all - now it's really too late to bother), you absolutely have to buy a lens that has its own AF motor. That restricts your options within Nikon's range and outside of it.
The really dumb thing is that in-body AF is not really a bad thing: at least Sigmas won't front/back-focus.