First, return the kit lenses. One of them is already too much :)
You can buy a macro lens and - if you are serious about macro photography - you should. A 5X lens is expensive but 1X ones exist too and have more magnification than most point and shoots. This means that you will be able to image something smaller than what you could with your point-and-shoot. Tubes help but so far I found that they do not push the limit much before image quality drops and they are difficult to handle and focus gets harder.
However, if you are doing macro for product and documentary photography, what I recommend more is to buy an ultra-zoom with manual controls such as Canon SX-series or Fuji S-series cameras. Look for models which can focus to 0cm (zero) from Canon or 1cm from Fuji.
The difference is that the depth-of-field in macros from an ultra-zoom is much greater than a DSLR with macro lens (or other macro contraption) and therefore much easier to focus and show an entire subject (similar to your P&S except that with manual-controls you can make sure the depth-of-field is truly extensive by stopping down the aperture to F/8 on Canon or F/11 on Fuji.
Your passing question to buy a Nikon is truly about of the blue. It is still a DSLR and you still wont have a macro lens for that price, so there will be no improvement regarding macro photography.
What you need to do is decide the kind of macro photography you want. If that is extreme closeups and shallow depth-of-field is part of the look, then upgrade your lenses one way or another (extension tubes, reversal, macro lens, etc). If you want to show something small completely in focus with ease, then get an ultra-zoom. Both the Fuji HS30 EXR and Canon SX40 are great choices.