Dave,
I'm thinking that as far as possible A/C savings go, as long as your level of insulation is currently adequate(?) you may be best off with these?
Insulated windows (with black out shades! Use the shades!)
Whole house fan-which allows you to pull the hot air near your ceiling up and out into your attic.(Leave the bedroom doors open to allow air to be drawn out of them too!)
And last (perhaps most important!)
The whole house fan pulling hot air out of the house displaces the hot (usually 150 + degree air) out of the attic. This can be aided greatly by powered attic ventilators.
If you think about it, it's much easier to get rid of the super heated air in the attic, then to try to insulate against it's effects down in the living area. Or to try cooling it down?
YMMV?