Where is the WiFi antenna located now that you've moved the system over to a new case? If the antenna is now inside of the new case then the case is acting as an RF shield blocking/reducing the WiFi signal from reaching the antenna.
The antennas were always inside the case. The wireless card isn't so much a 'card' but more of a processor looking component on the actual motherboard. The new case has one antenna inside the case and the other one wiring out of the case onto one of the screws of the expansion slots on the back of the case. Do you know which type of wifi card I'm talking about? It literally does look like a processor placed on the motherboard with two wires coming out from the top which are the antennas.