Insulation always helps. You can't go wrong insulating a house unless you screw up the vapor barrier.
In an attic, you can blow in more insulation - either Greenfiber or pink fiberglass. It takes two people to run the blower (one to operate the hose end and one to feed the machine), and both will want to be dressed in tyvek suits and will want to be wearing respirators and goggles -- if you are willing to spend the money, full face respirators are best. Don't just get dust masks; they don't work. Not wearing the tyvek suit is a foul; you will itch for days and your bed will itch for weeks no matter how much you shower. Not wearing a good respirator that is properly fitted (And I'm NOT kidding when I say that mustaches are OK in this department, as are goatees, but the bearded need to shave down to a goatee for a proper fit) is a serious foul because you can't itch inside your lungs to get the fiberglass fibers out. So you just get cancer instead.
Or if you don't want to go there, you can just roll out more batt insulation over the blown-in insulation.
But before you do either of those things, buy a six-pack of those cans of spray foam, and wearing rubber gloves because that stuff is evil on skin, find all the places that wires come up out of the top of walls, all the places where A/C vents penetrate into the attic, and all the place that plumbing goes down into walls, and fill up the holes with spray foam. This keeps air that's in the house from going up into the attic, or hot air in the attic from overpressuring when the wind blows and coming down into the house. For bonus points, also get one of those tent things for your attic hatch. It seems like a dumb idea, but then...
The main thing you need to make sure if you're "no A/C people" is that you maintain good attic ventilation. This means that air should be coming in through the soffits of your house and leaving through vents in the peak of your house -- either turtle vents, turbines, gable vents, or ridge vents. The right combination of these things depends heavily on the design of your house and where you are in the world/country, and you need to have a professional in the roofing or architecture trades tell you what's right for your area. The one thing that's uniform that everyone screws up when they have blown-in insulation is that they block the soffit vents. You need to put baffles in there. There's a lot of cheap baffles that you can buy off the shelf at Home Despot or bLowe's, but they suck and are hard to put in if they don't already exist. The really good ones are called Berger Accuvents and cost a little more. They have to be ordered at the contractor's desk or online.
The thing that most people don't realize is how much having a ton of insulation in the attic will quiet your house if you have a single story house. Neighbors have barking dogs (or, like me, geese)? They go from "nuisance" to "unnoticeable" when your house is properly insulated and sealed. Having filled my attic with insulation to R-60, replaced the wall insulation (was R-9), added a rainscreen siding build with proper sheathing and housewrap, and replaced the windows, I can't even hear the geese in my neighbors' backyard until I go outside. Geese are LOUD. It's a huge quality-of-life improvement that also pays off in reduced energy costs.
Mustache tip: If you're blowing in insulation, you use a machine to do it. The machine rental price is usually waived if you buy enough bales of insulation material. Most people will need the minimum to get the machine rental waived. However, if you don't, you can buy the minimum (usually 20 or 25) and then return the ones you don't use when you return the machine; they don't charge you retroactively for the machine rental as of the last time I did it early last year.
