To be honest, I'm not a huge fan of having the modem directly connected to a PC. I know it has router capabilities, but a true dedicated router will provide better security IMO. They can be had cheap, especially since you don't need wireless. However, you can get a cheap WiFi router with 4 ethernet ports for about $30.
Here's a link to one with 802.11N capabilities and a 4 ethernet ports for $30.
http://www.bestbuy.com/site/D-Link+-...&skuId=9706754
This will resolve all your issues, provide better security, and give you Wireless capabilites should you ever want them in the future.