This has been addressed before, but I can't remember where. The major problem with a 64-bit server is that the client is 32-bit because the game is 32-bit. This means all the networking needs to be 32-bit, which means most of the data types need to be 32-bit to be sent correctly. A 64-bit server is, of course, possible, but would require a lot of conversions and possible loss of data when things get sent over the wire.