Windows NT 4.0 and VPN

We have a Windows NT 4.0 computer that is our VPN server. We currently have it setup for 8 pptp ports. The computer is a Compaq Deskpro 4000 300MHZ. I am needed to add more pptp ports. My question is what will the impact on that computer be by added my ports? How many can I use before the computer is overloaded? My understanding is that it is just a pass through so system resources are not high. Is this correct? If it is then it should not matter how many ports we use? I found many docs on setting up VPN but none on suggesting the hardware to use to support the number of ports used.