Proxy for Internet Connection

We have Two Lease lines with two Cisco Routers,we are in the process of shifting the 64k internet connection to 128k, the existing settings of DNS servers of ISP is defined on the DNS tab of TCP/IP properties and the gateway address is given by the DHCP, now we have another address for the gateway for 128k internet connection.
I want to keep 64k Internet connection as backup.Now I want to use Proxy server for user permissions, caching, to protect LAN
My question is How do i configure these two gateways by using proxy server, or DNS server, and how do i protect my LAN from Internet.Waiting for your professional answers.