Tricky one without a little more information. I assume the two computers are 50ft apart with two walls between them. I also assume you have a broadband connection, if so is it cable or BT type?
Ideally you want the Wireless Router (Modem) to be between the two, thus the signal would only have to penetrate one wall in either direction. I would also go for MIMO (Multiple Input Multiple Output) wireless adapters - more chance of a signal getting round the walls.