NAT is of two types:a)Static and b)Dynamic. Since you want to use two private ip addresses internally you have to configure DYNAMIC NAT on your cisco router.Access lists are also to be configured as a part of NAT process.Following commands are to be given in order to configure the Dynamic NAT:
1. enable
2. configure terminal
3. ip nat pool name start-ip end-ip {netmask netmask | prefix-length prefix-length}
4. access-list access-list-number permit source [source-wildcard]
5. ip nat inside source list access-list-number pool name
6. interface type number
7. ip address ip-address mask
8. ip nat inside
9. exit
10. interface type number
11. ip address ip-address mask
12. ip nat outside
For examples of NAT configuration refer:
http://www.cisco.com/en/US/docs/ios/ipaddr/configuration/guide/iadnat_addr_consv_ps6922_TSD_Products_Configuration_Guide_Chapter