Can I connect an adsl-modem to 1720's ehternet port , and make 1720 as a NAT-gateway ? Which IOS sould I choose ?
Yes you can. Obvioiusly in this case you would need atleast 2 ethernet interfaces. One to connect to your PC LAN, while the other to connect to the cable modem LAN. You get one 10/100 port onboard of the 1720. While you will need to add an additional ethernet port using a WIC-1ENET card. THis is a 10 MBps only card. You can enable NAT on the 1720. You can enable NAT even on a IP only version of the IOS. If you have enough RAM/FLASH, you can upgrade to a 12.3.2(T). Check for the latest security advisories, released by Cisco, and upgrade to the right code.
http://www.cisco.com/warp/public/707/cisco-sa-20030717-blocked.shtml - gives info abt the latest security advisory from cisco.