What is your attic like? can you get to all the corners up there? pulling up the fiber glass and getting rid of it is going to help alot when you add new insulation as the fiber glass is going to allow air leaks from cracks and gaps and holes from wiring to keep causing problems.
What is best is to pull up the fiber glass and air seal with expanding foam, you can get cheap cans that are hard to use or you can get gun foam that uses larger cans with a application gun, or you can get a big can, it's easy to wince at the price of expanding foam, but there really isn't anything that does better in air sealing an old house, so to air seal with foam like this you find each and every wire that goes in to the attic and seal around it, same with vent pipes and duct work, light figures and chimneys need to be build up around with fire proof rock board or whatever meets code in your area, some recessed light fixtures are safe for contact with insulation, others are not, so make sure you know what you have so you don't start your house on fire! you can also get fire stop expanding foam that should be safe for around brick chimneys and some insulated double wall chimney pipe, but again, check to make sure that what you are using is safe for the application.
After you have everything sealed up make sure that your attic has enough venting from the sofets and that there is some sort of ducting to keep those vents from becoming blocked when you add insulation, a poorly vented attic will hold heat in the summer and hold condensation in the winter, it will also shorten the life of your roof costing you more money in the long run.
After your attic is sealed and vented then you are ready to blow in your R60 insulation and be done in the attic.
If your house was part of a mass development, then pull some trim off a window and check around it, see if there is insulation between the window and the framing of the house, fiberglass does not count as insulation, if you want to know why I say this then pull out a chunk of fiberglass and check out how much dirt it has been filtering from the air that is been passing around the window or door, foam is again the best choice of around windows and doors as it insulates and seals from drafts.
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Originally Posted by Daox
That being said, I recently went to a speaker who talked about doing deep energy retrofits on existing homes. Their target was ~70% reduction in energy usage and they were achieving it mainly by putting 4 inches of polystyrene on the outside of the house and then residing it. They of course did air sealing along with that via tyvek and other means.
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I checked out that first house they did the 4" of foam on while they were working on it and it was pretty impressive! it's about two miles from where I work.