Well Steve you came to the right place. There are basically three types for you to consider. Fiberglass, cellulose and foam. All three have both benefits and drawbacks. And choosing them is determined by many factors that need to be considered. I will start with fiberglass.
This is fairly easy product to work with. Its something that most homeowners can do on their own, thus saving on installation costs. It has the lowest "R" value per inch of any of the three products. And is reliant on proper air sealing to do its job. (more on that later) Cost per square foot is also the lowest of all three.
Blown in fiberglass which seems to be the new products that Home Depot is now selling is also a DIY type material much like that of the Cellulose described below. The installation is about the same, only you need more of it per inch to get the same R value as the Cellulose material.
Cellulose insulation which is really ground up paper product that has been treated with fire resistant chemicals and in some product lines chemicals that resist insects come in second on the list. It can be installed by the average home owner, but it will take two people to do this and a certain amount of pre-insulation prep work is required. This prep work included sealing off with baffles all soffit areas, Construction of boxes around ceiling lights to prevent direct contact with the fixture. Care must be taken in this area as overheating of the light fixtures often occurs if not enough clearance has been obtained around the fixtures. A wall of sorts must be constructed around the hatch or stairs as several inches of the material must be installed to deliver a R factor of around 30. Its dusty when its first applied. Can be installed directly over the older stuff. It does bridge the wood joists so no ghosting of ceiling joists occur. All electrical boxes and spice connections for cable and phone must be identified by markings on ceiling so you can find them later on if need be. And care must be taken when walking around in attic once done so you do not step through the ceiling. The application is pretty simple otherwise. You have your helper who is outside with machine keep putting insulation into it while you move the four inch hose around as it comes out. A two way radio is a good thing in case you want them to stop the machine for what ever reason. Yelling just does not work.
Cost of this material is middle of road. Most companies rent the machine for free once you purchase around 10 bags of insulation, but each bag runs around $15 or so. You will need to measure out the entire attic area to determine your final cost.
Foam is by far the best system there is. It of course comes with a price tag of around $1.10 to $1.40 per board foot. 12" x 12" x 1" However foam has the highest R value per inch then any other insulation available. Its truly not a DIY material,. although kits can be had through Tiger Foam.com and a few other places. Their cost is about the same for material alone as it is for having it done professionally with labor included. The biggest issue with the kits are you really never get the volume of foam that they claim. In any case six inches of foam can get as high as R-32 which would take many inches of the other materials to achieve. Air sealing is performed as part of the foaming process. Can lights etc would be boxed off same as the blown in stuff. Old insulation will need to come out as well. One big benefit of this type of insulation is that its pay back cost comes in around six or seven years after paying it to be installed. Still another reason to go the foam route is that combined with foaming the attic, you do the sills in the basement. This stops air flow through the walls which robs the house of heat.
Still another method of using foam is to do the roof cavity and not the ceiling. If you have AC ducts in attic this is the best method. Saving you on the cooling cost by reducing the equipment load by as much as 20%.
You remove all ventilation in attic, it stops all ice dams along roof edges. Makes the rest of the home warmer by preventing air currents from pulling heat and cooling air out of the house. Stops moisture in attic which results in mold.
The draw back expensive. However depending on where you live, many states have rebate programs that offset the cost of installation to a point that even with loans to complete the pay off, you end up seeing lower costs every month that offset what you were paying before you did the job.
Regardless of what type of insulation you choose. You must consider air sealing as part of the job. Air movement robs the insulation's ability to keep the heat in. As air blows through the insulation it pulls the heat out with it. A good example of this is in this photo attached. You can see the dirt trapped in the fiberglass as air moved though it. In effect the fiberglass insulation acted as a filter. If you currently have fiberglass in your attic. Pull it up where ever there are walls below. you will ind the same effect. All signs that air leakage occurs and removes the insulating ability of what ever is installed. This is what makes foam such a good product. It air seals and insulates at the same time.
Can't add much to Woodbrige's thorough dissertation, except to note that the cellulose is probably the "greenest" insulation option, as much cellulose insulation is largely recycled material.
I enjoy seeing your enthusiasm, Woodbridge. It is difficult to wrap my mind around all the concepts. We currently have blown-in insulation. But I keep a lot of things in the attic (holiday decorations, furniture, kids stuff, etc.) so I have placed planks up there to make a floor. I was told this was bad. I am in the process of removing most (but not all) of these things. I would still like to keep some of the "flooring" I put up there for storage. What do you recommend as far as additional insulation? Yes, I have found some dark spots in the existing insulation. We have a new roof with a ridge vent and lots of extra vents were added. We used to have an attic fan but the roofer said it was a waste and took it off. This is all so confusing! I can't afford to put a lot of money into this but our house is very hot during the Georgia summers.