my air conditioning only gets as cold as the air outside unless I start driving around 70mph.  Does anyone know what that means?  

Bring it to a qualified A/C shop. It's almost impossible to diagnois somthing like this over the computer.  

There are a few factors that can do that.  Low Freon, requires higher compressor speed to acheive a sufficient flow of freon for cooling. If low, why? Filling it up will result in it leaking and back to square one unless fixed.  Blocked condenser (the radiator thing in front of the radiator), if there is anything blocking the air flow through this it will reduce the effectiveness of the A/C cooling (not to mention engine cooling since the radiator is right behind it).  If you have a robust cooling system in the car and the front is partially blocked it's possible (though unlikely) that the A/C system is just more sensitive to blockage than the radiator.  By blockage I mean things like cardboard, paper, plastic bags, even turned off electric cooling fans, anything that keeps air from effectively going through the front of the condenser.  If it's determined that your freon levels are good and the condenser is clear if you don't have one an electric fan that comes on with the A/C system would probably help.  

If the freon level's good and the condenser is clear, you could either have a blocked/clogged expansion valve, or a bad pressure cycling switch.

Everyone's recommendation of taking the car to an A/C shop is a good one. A shop can hook the gauges up to your system, read the pressure and see what's going on.