First off does the server support 64-bit installs? Does the IBM server support Windows 2008 server?
What does this server currently do?
You would need to format the partitions on the server then install Windows 2008 server R2.
Here is a guide on migrating to 2008 server.
http://www.experts-exchange.com/OS/Microsoft_Operating_Systems/Server/Windows_Server_2008/Q_23665224.htmlIf your current DC is 32-bit make sure you use adprep32.