limited amount of users on server 2003 standard

Yes. Small Business Server is limited to 75 users.

http://www.microsoft.com/windowsserver2003/sbs/evaluation/faq/prodinfo.mspx

Q. How many employees is SBS 2003 R2 suitable for? When would you recommend a more robust solution?

A. SBS 2003 R2 is targeted at businesses with up to 75 users or devices. When a business gets close to the upper limit of 75 users or devices, it’s time to consider moving to the standalone versions of the SBS product technologies: Windows Server 2003 Standard Edition, Exchange Server 2003 Standard Edition, and SQL Server 2005 Workgroup Edition. These solutions will run on midsize hardware (four processors, 4 GB RAM for Windows Server 2003 Standard Edition) and are capable of intricate Active Directory configuration and management functions. If a customer has invested in SBS 2003 R2 and is at or near the 75-user or -device limit, consider investing in the SBS Transition Pack, which allows them to convert to standalone versions of the software and move to hardware that will scale with the workload. For more information on the Transition Pack, visit http://www.microsoft.com/WindowsServer2003/sbs/techinfo/planning/transition.mspx.