Reading th thread I get the impression that your not using a tripod for these shots (just the impression). Ir your going to start going beyond 1:1 magnifcation in macro, whilst its possible handheld you will find it a lot easier (and more accurate) using a tripod based setup - its more bulk and setup I appreciate, but it would give you more control and (in the long term) be quicker to use.
For a tripod setup a good solid tripod (manfrotto) is a must, then you will need a tripod head, for macro I would recomend the Manfrotto Junior geared head - its not cheap but it allows for rock solid and very fine control over the positioning of the camera, something you very much need for macro work - you will then need a focusing rail to help with focusing your setup - I would go for one of the
ebay focusing rails since they are more affordable over the Novoflex offical make ones (which are very expensive).
After that lets talk lenses - your work appears to be dealing with flowers and fully static subjects indoors, so a shorter focal length macro sounds like a bonus for you over a longer focal length macro (in the full 1:1 macro prime lens range the longer the focal lenght of the lens the more distance you have between subject and camera - important for insects, less so for flowers).
For this I would say go for the Sigma 70mm macro - I recomend this lens because it has full teleconverter compatabilty with sigma brand teleconverters (its not listed as such but it does fit them well, I have done this many times with mine).
Teleconverter compatability is important as adding them wil give you a boost to your magnification factor without affecting the focusing distances (essentailly you get closer). A 2*TC will let you get to 2:1 magnifcation - note that there willl be softening of the end image as a result of the incrased glass of the setup, but its still workable.
I would also recomend you to look at the Raynox series of macro diopters/lens attachments/filters such as the DCR250 since they offer you (again) an increase in magnifcation, though this time by making the lens focus closer to the subject - combined with the TCs you can get some impressive magnifcations
I have some examples here - there are notes on the shots (most of them) explaining my setup
Bee and other macro raynox experiments 1000 - a set on Flickr
IMG_0048 on Flickr - Photo Sharing!
Note that whilst they were done with the Sigam 150mm, the 70mm will still give you the same image magnifcations (the same shot framing) the only difference being that the 70mm will require you to be a lot closer to the subject to get the shot - a problem for living insects, less of a problem for your plants.
I will point out that the MPE65mm is generally the sharpest once you start going into the higher magnifactions, but its a very hard lens to get to use. Also once your going beyond 2:1 in any setup you will need to get a torch or similar to give you additional lighting when your focusing your shot - the viewfinder image will go very dark and can look very soft as well even when the shot is in focus - additiona lighting for focusing is important and its why the MPE65mm is most often used with the canon twinlight flash as it can provide a focusing light.
As a final point I would also recomend looking into focus stacking since depth of field becomes very tiny when you get into the higher magnifactions and sometimes you can't get all the shot in focus that you want - though if you stack a series of shots by setting the focus to manual and moving the camera a tiny bit close for a series of shots (using a focusing rail) and thus giving you a series of shots which in total cover the whole area you want in focus. Then using (free) software like Combine ZM you can stack all those shots into a single composit which will link up all the sharp points to give you an inreased depth of field in a single image - it will also give you a boost to sharpness and reduction in noise as well
An example from 5 images stacked into one
stack5 on Flickr - Photo Sharing!
Also something that you might bare in mind, but there are adaptors to allow cameras to be connected to microscopes as well as dedicated cameras for this task. I don't know of the image quality possible from one of these setups, but it might provide you a cheaper way of getting the high magnifcation shots your after.