Install & Configure a VPN server that assigns real/private IPS to vpn clients, providing internet access to the clients via vpn connection

I have a live production CentOS 5 server hosted in NY. I need a vpn server daemon/service running on that server, configured to provide internet access to connected connected clients. The CentOS 5 server has a range of dedicated IP's that I would like to assign to its vpn clients, additionally there are other clients I want to assign a private IP to, those clients would still get internet access via nat.

Additionally if there is a third party tool that allows simple web based management of users, bandwidth and transfer limits would be a bonus.

Things I cannot do: Recompile the kernel, Reboot the Server, introduce system wide changes that would affect the current live services: LAMP & SSH

Note: Solutions that follow an RTFM trend will not be accepted.  Please be as detailed and concise (reasonably) as possible. Also keep in mind detailing the basics such as how to use vi, and yum or package managment tools will not be needed. Im a fairly confident *nix sysadmin, unfortunately very weak when it concerns advance networking and vpn.