LAN and net access if you have cable/dsl:
Any cable/DSL wireless router box, cost is $20-100 depending on the model, Hot Deals has listed several in the $25-50 range.
If you're still on dial-up, either
(a) look for one that has a modem / serial port
(b) get a wired one that has a modem port, plus get a wireless "accesss point" (AP) and plug it into one of the wired ports
(c) use the wireless one as just a hub (not router) and set up Internet Connection Sharing on the downstairs PC.