I think you have 4 choices, extension tube, 65mm MPE, 100mm and 200mm. The 65mm can go up to 5x lifesize I believe. The 100 and 200 to 1x, but you gain working distance (and autofocus so you use them as normal lenses). With the 65mm, as stated above, you will likely need rails and a macro flash and you are looking at a huge investment. The extension tubes will allow you to use whatever lens you have. If you are just starting out with macro photography, that will allow you to play around and see what is most important to you (ie. magnification, working distance, DOF, etc.)