I would reccomend a major name brand 802.11g wireless router.. and a pci wireless 802.11g card for the downstairs desktop and and a pcmia (cardbus) card for the laptop and the upstairs machine can just use the regualr ethernet ports on the router.....
would be the easiest way.....
see pratciallynetworked.com for more information abotu setting up wireless netowrks and don't forget to enable WEP on the wireless cards
BTW does the cable/dsl modem use ethernet or USB or both, because you will need the ethernet ( otherwise things get MORE complicated )
also see
http://kiddspc.com/wep.html for a wep key generator
if 802.11g harware costs too much for your tastes try an 802.11g access point and just get 802.11b cards. the only difference is really that the cards are only capable of 11mbps instead of 802.11g's 54mbps