Tamron is a bit free with their use of the word macro, that lens can magnify 1:3.9. It's often been that lens manufacturers would use the term "macro" when a lens could achieve a magnification of 1:4 (meaning a 4 inch object would cast a 1 inch image on the sensor). To compare, the DA 50-200 can do 1:4, so you wouldn't be gaining much "macro" with that particular Tamron.
A "true" macro will do 1:2 or preferably 1:1 and is normally a prime lens. Since you have the DA 50-200, I would recommend getting one of these. Or perhaps the Sigma 70-300 APO or Tamron 70-300, both of which can do 1:2, about as much as you can get with a zoom. I orginally bought a Phoenix 100mm macro lens that is no longer made, but can often be found on ebay. It's got excellent optics for a cheap lens (it's made out of rather light-weight plastic). Since I quickly found out that AF doesn't always help you out when shooting macro, I ended up buying an unsold Vivitar 105mm macro lens, which is auto exposure/manual focus and have been very happy with it. Check out www.keh.com
and the used lens departments of B&H and Adorama for high quality manual focus macro lenses.