With 2000 vintage toilets they will be 1.6 gpf. I've got no strong preference either way with regards to dual flush. However, I'm not sure how much water you will really save by making such a change. It depends on the ratio and how often you have the need to double half-flush of full flush. The studies I've seen indicate some reduction with dual-flush vs. a 1.28 gpf HET.
How well do your present toilets work? What do you like/dislike about them? Are you to the point that your really don't like them and want them gone? Mine are old Gerbers with small waterspots. (I have no doubt the Caroma flushes well in comparison to these old things.) I had to modify one of the Gerbers to 2.7+ gpf to get a decent flush out of it. They really don't have much capacity, especially for paper. I haven't had to plunge any in several months, but that's because of the modification and quite a bit of training of the kids so that they use less paper and do intermediate flushes after a few sheets of paper.
The problems I've heard about with the Caroma's is that they have the small waterspot (mess and/or smell) and they tend to spray/atomize/mist some contents when flushing.
The dual flush will give you some reduction as would other high efficiency toilets, but neither will result in a large number compared to a working 1.6 gpf. The HET flush is 1.28 gpf vs. the standard 1.6 gpf. Dual flush will be 0.8/1.6 vs. 1.6 all the time.
As far as home water usage goes, unless one has the much older large flush volume tanks, big reductions are most easily made in the shower with lower flow showerheads (as in less than the 2.5 gpm standard limit). That depends of course on finding one that you like the performance of. The upside of this is that not only is it inexpensive and easy to change a showerhead compared to a toilet, but it also saves water heating costs whereas toilets don't. Unfortunately none of my local stores carry low flow showerheads, so the search has been online reviews and purchases.
