Water Heaters - most efficient water heater?

Our water heater just kicked the bucket. Being environmentally minded people we're interested in finding one that conserves energy and water. I have looked into tankless units, but some people seem to say that they don't save much, if any gas. Most of the standard water heaters we are seeing at the big-box stores are all about the same with efficiency ratings in the upper 50s. I have "heard" about more efficient tank-style water heaters, but I can't seem to actually find any anywhere.

We live in Madison WI so it gets pretty cold in the winter and have a gas hook up. Its just my wife and I, and we figure on about 10 showers, 2 loads of laundry and 1-2 runnings of the dishwasher per week. We have a low-flow shower head in the shower and a few other water saving features, though our dishwasher and washing machine are pretty standard. Right now we have a 40 ga tank and never run out of hot water.

Considering our low use of hot water, what would be the most environmentally sound option for us? We are on a tight budget right now, but we might be willing to invest in something if it will pay off in the long term.

Pay a visit to your electric power company local office. I know you have a gas hook up, but they offer extremely efficient water heaters, programs for rebates, possible free hook ups, etc. Ours has one that looks like a miniature atomic bomb, with 4" of foam insulation surrounding it. They also warrant them very well. With the rebates, etc. it could be a better deal than trying to buy one and installing it yourself from the big box stores. Just a thought.