You are nuts.
OS X Server is good with OS X clients. If you plan to continue to support the 100+ Windows clients, and they are domain members, then stay with Windows.
If you are going to replace the Windows clients with Macs, then the discussion is different.
And don't get me started on the superiority of Exchange (despite its complexity) over Apple's groupware solutions. Let's just say that using iCal server and Address Book server on OS X will exclude Windows clients from participating unless you change all your tools. Or you would need a product like Kerio.
I don't think you are nuts for wanting the über-stable system of Mac OS X. I do think that Strontium90 is right when he implies that you should keep the windows server to service your windows clients.
You could however set up an email server that you connect to your AD-domain and tell the Mail server to use AD authentication for it's service, leave the old one running, install the mail account for the new server at the clients and let them manually transfer all email. When finished point your MX record with the lowest (10) priority to you new server (mac) wile giving your old server (win) a higher (20) priority to your windows machine for backup purposes.
What services do you run anyway?
Thanks for the comments guys. I really appreciate them. Unfortunately we probably wont be able to get away from the Windows PC's for a long time. We recently invested in a lot of new Windows PC's for our users. I want our network to be simple. I dont care about Group Policy, Distributed File Systems, etc and many of the other features Windows 2008 R2 has to offer. DHCP, DNS and a nice User control center is all I need. We do use Microsoft Exchange 2007, but I heard the new OS X supports it in some way. I guess since you guys recommend not going the way of OS X becuase of all the Windows machines, I completely understand. Just thought it might be possible. Thanks, Pete
Instedd of using MS AD you can use Keberos as autentification server. Then it dosent mather wheter your user have Mac or a boring MS PC.
Here you have a link to howe you setup a server and clients.