You could look at something like the Nikkor 40mm 2.8 macro. This would double up as a pretty fast standard prime lens.
But it really depends what sort of macro shots you want to take and what focal length would be useful to you for general photography. If you plan on shooting insects like butterflies, you will most likely want a longer focal length macro upwards of 90mm to give you a better working distance that will mean you are less likely to spook them. Something like the Tamron 90mm, Tokina ATX 100mm or Nikkor 105mm I believe are good and would act as a portrait lens too but might be too long for your other needs.
Edited: I wrote SIgma 100mm when I actually meant the Tokina 100mm2.8. I haven't used those lenses but I've heard good things.