Upload & Sell: Off
Are you shooting DX or FX? from your profile, you've got a D300, which is great! But dj dunzie is right, you have to decide which focal length suits you best first.
I've had a lot of success with the 105mm focal length in the past with shooting casual macro and doing portrait work, both in FX/film and DX. However, there have been times, especially on film/FX that I wished it was longer when doing some macro work.
It really depends on you. But I will second that Sigma makes some sweet macro lenses, and a new 180mm is on the way.
(FYI, I have a Nikon 105VR, but wouldn't hesitate to pick up a Sigma)