Installtion and Management questions

mp

I am pretty new to Navision installation and management.
We have Navision 2.6 installed in our W2k server and around 15 clients access the Navision database. The database is .fbk file.

We are planning for an upgrade to 3.7. We want this upgrade to be smooth and easy. I am planning to get a stand alone machine in to the network and install 2000/XP Professional and want to test out the 3.7 version and give some time for the users to get a feel of 3.7 but still they will be using 2.6 for regular activities.

Once they feel comfortable I want to switch them over.
Also during this upgrade process, I want some time to upgrade the w2k server to 2003 server.

Questions
-Does navision require a server OS.
-If not can a 2000/XP professional handle 15-20 navision clients.
-How will the license issue affect when we test out 3.7.
-What are other key points to consider during a version upgrade.
-Currently we have one server and database is running. I really do not have any idea what to do incase of an emergency when our server goes down. What would be best ways to minimize our down time.