Absolutely return the 40mm and buy the longer focal length! With macro, the longer the focal length the better (which is why Sigma makes a 150mm, Canon a 180mm and Nikon a 200mm macro lens). The extra working distance is invaluable as you are less likely to bump into your subject or cast a shadow on your subject. Also, the extra distance means lighting is easier and you are less likely to intrude on subjects that may be more skittish than flowers.
The extra $200 you spend now will be well worth it over the long term. In fact, I would even recommend springing for the 105mm Micro-Nikkor (I know, it's almost $900) as it is an "FX" lens ready for full frame. If, in the future, you wish to move to full-frame (and it appears Nikon may make that more affordable with rumours of a full-frame body lower in cost than the D800), you will already have an excellent macro lens. In the meantime, with the 105, you will have the equivalent of a 155mm macro lens.
The problem with photography is that there is always something better out there and always at greater cost. Finding a balance between present-day affordability and long-term usefulness is difficult. Good luck with the decision.