RJ-45 Socket

I purchased a laptop recently and so have decided to organise a wireless adsl router so that my desktop pc (Dimension 2400) and the laptop can share the same broadband connection. I chose BT Voyager 2110 Wireless ADSL Router. There are four wired Ethernet connections at the back of the router and I intend using one of them for my desktop pc. The instructions for doing so add: 'If your computer does not have an RJ-45 socket then you need to install a 10/100 Base-T Ethernet Network card'. There is a network adapter connection on the back of the computer so I was wondering if this is the RJ-45 socket. In other words, does the Dimension 2400 have what I require or do I have to install the Ethernet Network card mentioned above. This whole area of network stuff is new to me so I know little or nothing about this side of things. Your help would be appreciated. Many thanks.