Why should I upgrade

I’m running a NT 4.0 network with 7 NT 4.0 servers and 5 Quantum Snap file servers with 25 Windows 2000 clients and 10 Windows 98 clients. I have my own T1 connection/router, e-mail server and firewall. All users are running Office 2000 using Outlook and Joydesk for e-mail. The majority of my users are CAD users and run varius Autodesk applications on the network and a company wide accounting and time tracking database. Can anyone point out why I should upgrade my Network of clients to XP ? is it worth the time and pain for a small company like us today ? should I wait another year ?