Move Exchange to Diff Server w/ diff OS

There is a Technet article that covers this subject. I actually did the same thing you are facing about 9 months or so ago. It went without a hitch. The one thing I would recommend is to place your server in a workgroup initially and name it the same name as your existing Exchange server. Install IIS (if you wish to use OWA) and then install EX5.5. You can either perform a network backup of your existing Mail server and restore to the new server when you take the old one off-line. Or you can copy the files outlined in the Technet article to a network location and move them to the new server when you put it on line. Thats what I did and it worked like a charm!

http://support.microsoft.com/default.aspx?scid=kb;en-us;155216

Would you consider reposting your question in the Exchange interchange discussion thread, a TechRepublic discussion thread dedicated to Exchange admins? There your question would be seen both by experts who could answer your question and by novices who could learn from it. If you're interested, here's the link to the thread:
https://www.techrepublic.com/forumdiscuss/thread_detail.jhtml;?thread_id=117712