Bought one of these on Thursday.
yes, the D834 replaces the BT modem.
Once connected to the PC, you will have to access it and tell it your user/name, THEN plug it into the ADSL socket.
The paperwork is a bit sparse, but everything you need to know is on the CD Netgear supply.
You will have to tell all your programs to conenct to the www via the new 'always on' router, and not via dial-up.
It took me (as a complete network novice) about 4 hours to get everything set up on both PCs, but I went very slowly.