Windows 2000 has support for multiple monitors. Before getting into deep waters, check the HCL list for Windows 2000 and make sure that the video cards you are using are listed and that there are 2000 drivers. Unfortunately if you have multiple monitors working under Win98, the settings will not carry through the Windows 2000 upgrade. Windows 2000 uses a different multiple monitor architecture than Win98 and legacy video settings are lost. Sorry. Unfortunately the newsgroups are encrusted with messages about dual monitor support problems. There are a lot of video cards that will not work as secondary under Windows 2000 even though they worked that way under Win98.

Multimon Site dedicated to this topic.
How to Set Up and Troubleshoot Multiple Monitors in Windows 2000
Computer Runs Out of System Page Table Entries with Multiple Monitors
Problems After Installing Secondary Video Adapter
Problems Using ATI Rage Pro Turbo Video Adapter as Secondary Adapter
ATI LT Rage Pro Does Not Work when Used as a Secondary Display Adapter
Distortion Using ATI VT4 or Rage2C as Secondary Display

If you have Win95 or NT4, you can do this with specialized hardware from Appian Graphics or Matrox .