An orfice tube is just that, a fixed orifice designed to allow the high pressure liquid to expand to a low pressure liquid/gas combination. An expansion valve pretty much does the same thing, but modulates and regulates the amount of refrigerant passed into the evap coil based on the suction temperature. With just an orifice tube you generally had a large drier that was designed to catch any liquid that made it past the coil to avoid slugging the compressor and ruining it. If your system is being taken apart, you may also want to replace the condensor as well, as most modern day cars use the condensor as a filter and screen. The expansion valve is around $20, so definitely remove and replace.
I guess I don't follow why the system was disconnected for a radiator swap. Did you open your AC System when swapping the radiator? I wouldn't think that would be necessary, but I haven't swapped a radiator on this car. But would be the first I've heard of it.
Make sure to have your system properly vacuumed out. Air in the system will reduce efficiency and drasticaly reduce the life of the system. And if you are charging it yourself, never hook a refrigerant can to the high side. The high pressure side could be running between 250-350psi and would quickly blow the can and leave extreme refrigerant burns. In generally, I don't recommend too many people to work on their own AC system unless they have worked some form of HVAC before.
