the modem here is downstairs, and i want internet access upstairs without using wireless or running a cable through the house. could i buy another modem and plug it in to the cable upstairs for internet access? what are my best options?

I would say that would not work. I'm not sure who your ISP is but with Comcast for example, you would not be able to do this without two cable lines run to the house. That would be more complicated than networking and I'm not even sure that you could convince the cable company to do this.

There is a simple option to do this wirelessly that would not require a router. If you install a separate wireless card in the computer hooked up to the modem, you could share the modem connection with the wireless connection. Then on the second computer install a wireless card and connect the two in a peer-peer connection.

What OS are you running on the computer connected to the modem. I know sharing of the network works in Windows XP/Vista. I'm not sure about earlier Windows or Mac or Linux.

You could also do this setup with network cables too. Other than that you would need a router for more connections.