Exchange 5.5 to Exchange 2000

May be it's a bit complicated, but I think this is the best method to perform a "smooth" migration to W2K-E2K. Your users won't even notice you're performing a migration !

First, install your new server as a NT4 BDC, then promote it as PDC of your domain (server manager console).
Now, upgrade your PDC to Windows 2000. Since you come from a NT4 PDC, your W2K will be directly installed as a DC.
Using the E2K CD-ROM, install the Active Directory Connector (ADC) and configure it in such a way the Exchange 5.5 recipients are "connected" to Active Directory users located in the USERS container in Active Directory Users and Computers - ADUC - (by default). Note that Exchange 5.5 must be at least SP3 ! The purpose of that installation is to synchronize both Directory Services of Exchange 5.5 and Windows 2000. Now, you should be able to create a user in ADUC and, in the third step of the creation, you are proposed to create a mailbox...
Once everything's OK, install Exchange 2000. Specify you want to join an existing organization. Make sure your DNS-WINS resolution is in good shape...
Now that E2K is installed, you can move your users mailbox (ADUC --> right-click a user --> Exchange Tasks --> Move mailbox); you can also create replicas of your public folders. You also have to create SMTP connector to replace the Internet Mail Connector of E55. Just play with the connectors priorities to smoothly switch between the 2 connectors...
Finally, you can remove the Exchange 5.5 fromyour organization. Refer to technet article KB284148.

Hope this will help you...

Here's a smiple solution depending on the size of your network.
Export everyones outlook data to a pst file and import the pst file back into the Exchange 2k server. Do this in Outlook. This is a simplified solution but if you have more than 50 users.....it's a time consuming effort.
Good Luck!

Or a third, even more simple solution since you say you are going to replace the box, and are therefore doing a clean install... Use the "ExMerge" utility that ships with the product.
There is one version that loads on the NT machine and does a batch convertion of all ( or selected) exchange data to .PST files. A second version that loads on the W2K server converts the data the other way. Just be sure to make your user accounts first, so there are mailboxes available to receive the data.
Ihope this helps... there is ample documentation on the MS website. I just did this last weekend and it worked as slick as can be. Good Luck !