You want a LinkSys WET54GS wireless-to-ethernet bridge with built in 5 port switch.
With it you can create a downstairs LAN (using the LinkSys built-in 5 port switch) which the LinkSys will bridge wirelessly to your upstairs LAN.
With it you can create a downstairs LAN (using the LinkSys built-in 5 port switch) which the LinkSys will bridge wirelessly to your upstairs LAN.