I use the Sigma 105mm EX DG macro and love it, it is a great lens. I think you saw my thread about my first try with it
http://www.socalireefs.com/forums/showthread.php?t=2101
It can also be purchased for Canon or Nikon (or other brand bodies). Basically your camera body isn't going to be the difference maker, it's all about the glass you get. On a budget you'll do great with the XTI body and Sigma macro lens.
Here's my usual word of warning: dropping the cash on a DSLR and macro lens will not result in good pictures. Hopefully you are already familiar with SLR photography, if not there's quite a learning curve to shooting quality pics. Macro photography is even more difficult and tank shots are IME the most difficult. I highly recommend going this route, but you must be willing to be patient and committed to learning.....it's the photographer's skill that takes good pics not the equipment.