You'll need a Wic-1ENET card installed in your 1720, to connect to your cable modem. This is basically a 10Mbps ethernet card for your router. HOWEVER...this card will only work with certain IOS versions, so be sure you have or can obtain the proper IOS. Also, if your ISP issues you a dynamic IP via DHCP, you also need to be running a version of the IOS that lets you configure the interface with the "ip address dhcp" command. Finally, you need to configure NAT on your router, and set a default route to route all traffic out the Wic-1ENET to the internet.
As far as using the linksys as a wireless access point, you basically need to just use the linksys as a switch. This means when you connect the linksys to the 1720, run a straight-through cable from the fast ethernet interface on the 1720, to one of the 4 LAN ports on the linksys (don't plug it into the "internet" port on the linksys). you need to configure the LAN settings on the linksys with an IP address/subnet mask that is in the range your private LAN uses, and make the default gateway the IP address you assign to the Fast Ethernet interface on the 1720. You don't need to configure the WAN settings on the linksys, because you're just using it as a switch. You should then be able to connect to the wireless access point with your laptop, and surf the web. Just make sure the default gateway on your laptop is the Fast Ethernet port on the 1720 router, and not the IP of the linksys. You might have to turn off DHCP on the linksys, because the linksys router will probably identify itself as the default gateway when it assigns IP addresses. You might be able to configure the linksys router to give out the proper gateway, but if not, just turn off DHCP and assign the addresses manually to your PC's.
