Spruance - why do you want to go wireless, apart from this allowing you to move the router to the main linebox? Wireless comes with its own set of problems, so don't go down that route unless you need to.
It sounds as though the upstairs linebox is not the main one - the main one should always be the one which connects to BT's incoming line. Even though you are on a secondary, that is not necessarily a bad thing. You've already said that your existing Speedtouch is fine - a good router should be better than a USB modem, so it is likely that your performance with the new router will either be equal to, or better than, what you had before.
The issue of moving routers to the master socket is generally so that the secondary's can be disconnected for testing, or where the wiring to the secondary's is "iffy"! Since BT did the work for you, it ought to be done properly with the correct cable. In cases where the secondary socket is well wired, and a short distance from the primary, it is quite likely that you will see no improvement at all. I'd suggest that before you do anything else, you temporarily move the PC to beside the master socket, and compare performance with what you get at the secondary. If there's no difference, save yourself any headaches of going wireless.