i have never tested this, but the only way i see it working is by having at least 2 wireless cards running in Ad-Hoc mode. one of the computers with this card must act as an ICS server serving the internet connection to the other machine. that server is the only machine which needs a modem connected to it.
this method requires no router at all. i have never tested that out so i am not certain it will work. My recommendation is don't be a cheapskate. get the wireless access point or wireless router, you save some headaches.