Ravin,
The router will be configured with dynamic-maps for the outside so this is fairly well documented and would be hard to disrupt the internal/external traffic. Unlike peer-to-peer tunnel, which would use crypto ACLs to determine the traffic to be encrypted and could be disruptive if configured incorrectly. Check out the IPSEC/IKE Configuration Examples and TechNotes for examples on how to configure these.
Go to section below for about 4 to 5 sample docs.
IPSec on Router to Cisco Secure VPN Client
http://www.cisco.com/en/US/tech/tk583/tk372/tech_configuration_examples_list.html
NAT is not required to provide VPN services. The Cisco VPN client will require an IP, which can be static, IP Pool, DHCP, etc. A no NAT example is in the link I provided above.
Please rate any helpful posts
Thanks
Fred