http://www.cisco.com/en/US/docs/routers/access/cvo/880-870-cvo.html includes instructions for setting up a VPN, but I think the dynamic IP is going to cause you problems. It might still be possible but I suspect you may have to resort to IOS rather than doing it through the web interface.
This may not be useful if you're stuck with the Ciscos, but the Draytek routers, like the 2910, can be configured to use a "one way" VPN i.e. the end with the dynamic IP dials into the static IP. This avoids problems with having a dynamic IP, but it means the VPN can only be initiated from one end.
I have configured LAN to LAN VPNs using DynDNS names, and it mostly works though I have to say it leaves me feeling slightly uneasy.
Check http://www.cisco.com/en/US/tech/tk583/tk372/technologies_configuration_example09186a00801dddbb.shtml It explain how it works, just don't take care of the computer that connect with the cisco vpn client.
http://www.cisco.com/en/US/docs/routers/access/cvo/880-870-cvo.html includes instructions for setting up a VPN, but I think the dynamic IP is going to cause you problems. It might still be possible but I suspect you may have to resort to IOS rather than doing it through the web interface.
This may not be useful if you're stuck with the Ciscos, but the Draytek routers, like the 2910, can be configured to use a "one way" VPN i.e. the end with the dynamic IP dials into the static IP. This avoids problems with having a dynamic IP, but it means the VPN can only be initiated from one end.
I have configured LAN to LAN VPNs using DynDNS names, and it mostly works though I have to say it leaves me feeling slightly uneasy.
JR
Yes, this should be possible.
Here is the data-sheet for the Cisco 880 Series Integrated Services Routers:
http://www.cisco.com/en/US/prod/collateral/routers/ps380/data_sheet_c78_459542.html
As long as one of the sites has a static ip you can use dynamic map to configure the vpn. Details: http://www.cisco.com/en/US/docs/ios/12_3/security/command/reference/sec_c2g.html#wp1069489