question?

if there is a computer upstairs and one downstairs
is it possible to have them both hooked up to the internet
with a single modem(pref. dsl)?

 

Yes, you just have to have them both routed to the modem either through wireless router or drop a line down/up through the wall/floor.

 

The following collection of links might help.

Ignore the prices in the links bellow. The same or similar hardware can be found for 60%-90% less.

Link to: Basic Options for Internet Connection Sharing.

Link to: Hubs, routers, switches, DSL, LANs, WANs...?

Link to: Diagram of Broadband Connection with Cable/DSL Router.