If you get a printer with ethernet connectivity, then it attaches DIRECTLY to
the router and you only need the router and any single system which needs to print
to be powered on. Any set of systems will be able to share the NETWORKED printer.
Note: the USB port on the router WILL NOT WORK for this. That port goes to a
modem for the DSL connection in place of using an ethernet connection.
The printer with a USB port is designed for a direct connection to a computer,
not a network. Even using a USB-Ethernet cable will not turn it into a NETWORKED printer.
As Nodsu has already stated, you can get a small 'print server box' and the
connection would look like
router -- printserver -- Networked Printer
.......+---systemA
.......+---systemB
and all systems will share Networked Printer