wheel - 9:43 pm on Feb 9, 2010 (gmt 0)
as per johhnie.
Assuming you've got all the technical stuff done and moved to the new server, you can test it by assigning the IP address of your sites to the new IP, via your hosts file (you can search to find out where the file is). In other words, you force your computer (and just your computer) to see the sites on the new server, allowing you to test everything's working smooth. Once it is, then you change your dns setting and the rest of the world rolls over to the new server.