Does your desktop computer (the Gateway) connect to the broadband modem by an ethernet connection or a USB connection.  If it currently connects by an etherent cable, then you what you need is a wireless router like the Linksys BEFW11S4 or the Linksys WRT54G.  You would then connect the broadband modem to the WAN port of the wireless router, and connect the Gateway computer to one of the 4 LAN ports on the wireless router.  The laptop would connect through the wireless part of the router.  This would allow both computers to use the internet connection and you could also share files and printers between the two computers.  Note that both of the routers mentioned above would connect to the internet at the same speed.  The only advantage that the WRT54G has over the BEFW11S4 is for file and printer sharing.  The WRT54G would be faster on that aspect of the network, assuming that the wireless network adapter in the new laptop supports the wireless g protocol.

If your broadband modem only can connect to a USB port, it is more complicated, so let us know how the computer currently connects to the modem.

Steve