Are talking about using your server to be a PPTP end point or Terminal Services?

Windows XP is limited to 1 concurrent connection with 2 total cached sessions. If you want to run more Terminal Service connections then you need to switch to a Server with Terminal Services licensing installed.

Even though the code base is very simular in Windows 2000 and Windows Server 2000 just as XP and Server 2003, Microsoft purposely cripples the workstation versions to limit network connections so that you are forced to go buy their server products. No more than 10 computers can access a share on a "workstation" in a windows network.

Windows server products have RAS (Remote Access Service) that allows you to configure multiple PPTP and VPN connections.