What is needed to VPN

Hi,
i have been asked to install a hardware firewall so that users can VPN from outside. The thing is I dont know what is needed. We run windows 2000 server and a mail server as well. How many static IP do we need? I know one is needed for the mail server. What about the router firewall? what software do we need ?
Any help please, or any other info missing please say. Thanks.