If your adsl filter is coming out of a modem by the house entry point then yes its that simple.
If your phone line comes into the building and is terminated with a little white box (usually) and you have the filter attached there with no modem, then no.
Wall Socket --> adsl filter --> ethernet modem / router --> ethernet card (NIC) --> PC
Ehternet is not a linux thing, or a windows thing, ethernet is a standard protocal developed in the 70's for transferring data over a LAN (local area network). This is why its the best option for running your broadband connection.
What you are doing is creating a mini network between devices to join a bigger network.
EDIT: oops, forgot, yes an internal motherboard based ethernet device should be perfect. I use one on my desktop, and our backup server uses a £5 PCI card.