Graeme,
This is my approach to your querry:
Depending up on the Broadband [Cable vs DSL the former via Coaxial cable and the latter via Telephone lines)there are different ways to accomplish this.
Cable modem can be Internal but are most commonly External
If your PC is already equipped with either an Integrated or Add-on PCI Ethernet card you must get the Ethernet Cable Modem from the Cable ISP: the other choice is a USB Cable modem
To independently access the Internet from each of the Computers in the home network, you must have a Cable/DSL Router and you can choose from Wired Routers or Wireless Routers made by different manufacturers with varying price range.
If you have decided on getting a Wired Cable/DSL Router, you must next decide which PC you want near the Cable Modem and the Cable/DSL Router: the upstairs one or the one downstairs. Unlesss the house is Prewired for Broadband Internet, you need to pull some Cat 5 Straight-through patch cables between the Cable/DSL Router and the PCs located geographically at a distance from the Cable/DSL Router. The Cable Company is usually responsible for bringing the CATV cable to the Cable Modem they provide after connecting a Splitter at the Cable Box to which the ground Cable comes. If you choose the Upstairs PC, then they will pull the cable from the box to the Attic and then down between walls to your Upstairs room [which is an advantage if you decide to complete your Home-networking yourself as Cable Co tech has done the 'fishing line' placement of the Coaxial Cable to the Cable Modem and you need to retrace the path back to the attic along the same route for your Cat5 Straight-through (Patch) cable to avoid any 'obstacles' inside the wall and then down the wall to your Downstairs PC's RJ45 Jack of Ethernet Card :-)]
So, the things needed to complete the Home Cable Network of two PCs situated 25/50/100 feet apart are:
1. Ethernet Cable Modem from Cable ISP
2. Ethernet Cards either Add-on or Integrated on both PCs.
3. one 3'Cat5 Straight-through cable (not Cross-over )supplied with the Ethernet Cable modem. to connect the cable modem to the Cable/DSL Router [NOTE: the cable Company brings that to connect the Cable modem directly to the Single PC via the PC's Ethernet Card RJ45 Jack]
4. Cable/DSL Router
5. 3-7 ' Cat5 Straight-through cable to connect the Cable/DSL Router to the First PC which is closer to the Devices.
6. 25/50/100 Feet Cat5 Straight-through patch cable to connect the Cable/DSL Router to the Downstairs PC
If you want File and other Resource sharing between the Networkerd PCs, you may use the Software that comes with the Router.