A little about my house in Texas and a shameless plug for my clean energy
blog (gotta build those Google links). It was built in the 80's. Single pane windows. I'm not sure how good the insulation is, but I think the roof needs to be replaced in a few years. When that happens, we will definitely go solar.
We're running about $100/mo on our utilities now (gas, water, electric, waste) which is not bad. That will jump up in the summer, though.
I think we've done the easy efficiency things... (LED bulbs, efficient AC & appliances, bubblewrapped fridge, plugged drafts).. but I am interested in your ideas. The
blog talks about how we've put heavy curtains up instead of replacing the single-pane windows. It was a lot cheaper and easier, so we're pretty happy with that. Eventually we'll do the window replacement, too, but I expect it to cost several thousand bucks.