I know there's a floor insulation issue, but what about redirecting air, and other sources of leaks? Does your basement have vents in it? Turn them off and maybe consider getting bent covers that can seal in the summer. Close the vents on the first floor so that (the majority of) the cool air comes in at the second floor and sinks down. Weather-strip and baffle the basement door.

Also, see if your electric company offers a home energy assessment. It takes about an hour, and there are usually two tiers: free, and a second tier where you pay for a blow test. They can make some recommendations on making your home more energy efficient.