Go with Nos' suggestion.
Leave the modem in place and just get a Belkin, Netgear or D-Link (or other well-known name) wireless router and a wireless network card for the PC/Mac. It used to be better to have router and cards from the same manufacturer for best performance, probably less so now.
If the Mac already has wireless hardware, then you just need to set up the router and tell the Mac what to look for. There's also USB and PC-Card ones available if you have a laptop and can't retrofit wireless.
Don't forget to set up security settings on the router - WPA etc.