With a steam system you are limited to an 80% efficiency boiler. The most economical heating for our area is Gas Forced Air (Central Heating). With a central heating system, you do not have to boil the water into steam (which takes a long time, and a lot of gas). Forced air systems are on demand heat, you only use the gas when needed. Definately could cut costs over your old boiler by 50% or more. However, to install a new system, including the air conditioner, you are looking at @ $10,000+.
A new steam system, matched up with a programable thermostat would save you about 30% of your current gas bills.
Call these guys, they were inexpensive and great quality