How is the Sagem modem connected via a USB port or Via an Ethernet cable to the Lan port on the computer?
If via USB you will need to purchase a combine ADSL2/2+ Modem Wireless 54Mbps Firewall Router sample of the type required.
Linksys Wireless-G ADSL Home Gateway Router
3Com Office Connect Wireless 54Mbps ADSL2/2+ Modem Firewall Router
D-Link Wireless G ADSL2+ Modem Router
Using the above type routers you would remove your present modem and do a total new install
If via an Ethernet cable you can purchase just a wireless router, which you connect to your present modem via the Ethernet cable
Linksys Wireless-G 54Mbps Broadband Router
D-Link 54Mbps Wireless Broadband Router