Congratulations! (hopefully, fingers crossed for you)
Maybe you've seen it/posted there already, but definitely check out the
EcoRenovator.org website for more info/insight. EDIT: Went to the ER site after checking EM and saw you posted their.
There are a million-and-a-half things you can do, so a priority list will be of great benefit (especially if you include some dates/timelines). If you plan on being there for more than a decade, I would do a retrofit as soon as possible, so that ROI can be seen as quickly as possible. It will take some time, but be of great benefit to make a list of things to do, figure out cost, savings, ROI time and put it in order. Do the things with the shortest ROI and then make decisions on the other stuff. This might help you get started on a list:
Master ROI Table
Definitely change the water heater. You might want to look into on-demand heaters as well. Costs more usually, but figure out the calculations, it might be worth it. If you get a 'conventional' heater, look into some heat pump versions.
I would get a heat pump too. I don't know if they're available, but you may be able to find some that heat and cool (reverse the heat flow), which may or may not be of benefit for you (it would save space and be less 'stuff' to maintain. May or may not be more efficient).
I haven't heard of the waste water heat recovery before, but insulated pipes will go a long way.
Try this showerhead:
American Standard. It has three settings. The first one only uses 1.5GPM and the other 2 settings use 2GPM. When you turn the water off, no matter what setting you're on, it reverts back to the 1.5GPM setting, so there is no 'whoops, forgot' issues. Best of all, my better half likes the shower head, so happy life!
Try this faucet aerator. It is .5gpm instead of .75.
.5GPM Aerator. I found some on Amazon and other places too.
.8gpf toilets are really good! When doing a project for school, I came across these
Water Fill Diverters. They are designed so that you fill the tank faster, meaning less water in the bowl and wasting less water.
I don't know about the insulation in terms of taking out or leaving the old, but I would definitely look into increasing your r-rating. Call a specialist and ask lots of questions. If you add insulation to what is already there, putting it on the rafters may or may not help. Again, ask the specialist.
Being in Georgia, do you have a dehumidifier of any kind? That might not be efficient (one more thing to power), but might help keep the home safe from moisture (and more importantly, mold). You can do a solar powered one as well. (Check the ER site for that)
Good luck and fingers crossed!