According to http://www.dailygreen.com, which is a good starting point for finding more info., homes that are "green homes" are getting 4 to 11% more than other homes. I can't verify that, but, as an agent who is trying to sell homes in a slow market with too many overpriced, underconditioned homes on the market, I'm thinking that it might just help a home stand out from the rest. There are more and more people who are concerned about rising energy costs, and if you are going to replace an old system anyway, you might be wise to consider it. There are a few online listing services that specialize in green homes.
It is huge on the west coast, and it will make its way here eventually. I would definitely do my research, though.
If you can do the essentials that really sell a home, (updated kitchen and baths) and have enough $ to do some green updates, that would be the perfect mix.
I suggest that you consult with a local Realtor. Look at other homes in the area that are similar to yours and see what features they have. I can tell you that buyers are looking for VALUE, so anything that adds value (such as lower heating bills) a buyer will appreciate. However, there are tradeoffs.
How long do you plan to stay in yoru home? Long enough to enjoy the improvements?
Generally speaking the value of investments are recouped in bath and kitchen remodels, because who make the buying decision? Other necessary, but less noticeable investments, like the roof, plumbing, have value, but the return on investment is lower.
Are you making your home more livable? Or treating is like a flip?
A Realtor can help you work through those questions.
If the oil/boiler system is old, a simple home warranty may give them piece of mind. I would recommend doing more research on a home warranty plan for the boiler to make sure it can be covered.