Rather than take my word, or any other agent that's not in WY, I think you need to talk to some local agents. The National Association of Realtors has a saying, "All markets are local." What may work in my Seattle market may not in yours.
My wife and I love to meet with future sellers in advance of them making decisions like yours so that your time and money are well spent. Some improvements do bring a positive return on investment; others will sell your home quicker, but not necessarily for more money. Talk to a local agent who is successfully selling homes in your area and see what they think.
And how would this increase you home's value? Are you planning on converting your garage to living space? If so, there is likely much more to the process than this. Consult with a contractor or the building codes office.
http://www.remodeling.hw.net/2010/costvsvalue/division/mount Garage improvements seem to be around: replacing the door or adding a garage
Unless the buyer is a car buff or someone who intends to spend a lot of time in the garage or convert the garage to living space (hence the heated space), you may be want to put your money in other projects that will surely attract more attention and reap more benefits in terms of creature comforts and upgrades.
I don't think it's worth the hastle and expense (unless you can get some good use out of the garage before you sell.) Whether a garage is heated or not will very rarely, if ever, be a deciding factor in someone's home purchase. It's definitly nice to have if your a gear head or enjoy wood working, but most people look at the garage as a place to park there cars and store some things. I don't see the value of your home increasing at all by doing so.