The BizPreneur

How do I share an Internet Connection between two sets of devices on different floors?

I have a DSL Modem upstairs. It has four Ethernet ports. My computer is plugged into one of the Ethernet ports with an Ethernet cable. It connects to the Internet through the Modem.

But the problem is, my other computer, my Xbox 360, and my PlayStation 2 are downstairs. How do I get the Internet connection from the DSL Modem upstairs to work with my PC, 360, and PS2 downstairs? Do I need to buy a Wireless Router, put it downstairs, and have it wirelessly connect with the modem upstairs? I cannot run 3 Ethernet cables from downstairs to upstairs, mainly because it isn't very practical, but also because my computer room upstairs and my computer/game room downstairs are on opposite sides of the house. So how do I share the connection?