copied and pasted from another thread that I posted in:
the tamron 90 ($469) is awesome, but will NOT autofocus on the D40x
most people dont use AF for macro shooting anyways
if AF is worth $300 extra dollars, then get the nikkor 105 VR AF-S for $759
the nikkor would be a better choice if you ever plan on doing hand held macro as well (due to the VR)
i.e. out on a hike, etc
but still, the tamron 90 is not to be considered "inferior" by any means
but for $300 you get
1. longer focal length (might come in handy for macro-ing live subjects that dont like you to be so close)
2. auto focus on every nikon camera (even D40 and D60)
3. vibration reduction (if you dont have a VR lens, then you wont understand how awesome the VR is)
so you have to decide what is most important to you
if you are thinking that you might get serious about photography, then (just like getting a bigger aquarium) you will undoubtedly get a bigger camera body someday, which will AF the tamron 90
the sigma 150 is around $500 too, but I dont know anything about that