I developed a 22 home green neighborhood called "Eno Commons" here in Durham, finished in 2000. Used passive solar (window orientation, overhangs, thermal mass -- basically zero extra cost), geothermal heat pumps, metal roofing, low-e argon-filled windows on E, W, N and clear windows, argon-filled on south to let in the winter sun. Had an air-tightness standard on the shell tested with blower-door testing, as well as blower-door tested air ducts.
In NC, 6" walls aren't really needed: 4" walls with damp-spray cellulose insulation and insulated board sheathing give a good wall. Off-set studs are a great idea, but basically overkill in our climate. I would avoid fiberglass insulation, in any case. Lots of air infiltration and, as commonly installed, places where compression compromises R-value.
These features created homes that sold for less than the median prices at that time, yet used 60% less energy. My 3 BR, 2 BA house had a year-round average of $36/mo for an all-electric house (only other utility bill was water). And it was really comfortable--we just set the thermostat at 70 and left it.
The passive solar features qualify for tax credits in NC, too. I got a credit of $1800 the year I moved in.
I totally agree with Danase: Sealing, insulation and solar hot water are the big improvements from a cost/benefit perspective, and basically if these upgrades are folded into a longer term mortgage, they generate savings from day 1. Photovoltaics are totally cool, but still expensive.
If anyone has interest in seeing the features we used at Eno Commons, just google "Eno Commons" and "Green Building". the first link is the NC Green Building database entry for EC. There was also an article about the development in Fine Homebuilding April '99.