here are the added features in xp pro according to wikipedia:
The two major editions are Windows XP Home Edition, designed for home users, and Windows XP Professional, designed for business and power-users.
Windows XP Professional offers a number of features unavailable in the Home Edition, including:
the ability to become part of a Windows Server domain — a group of computers that are remotely managed by one or more central servers (many businesses that use Windows have a Windows Server and a domain)
the use of a sophisticated access control scheme that allows specific permissions on files to be granted to specific users under normal circumstances.
the Remote Desktop server, which allows a PC be operated by another Windows XP user over a local area network or the Internet
offline Files and Folders, which allow the PC to automatically store a copy of files from another networked computer and work with them while disconnected from the network
the Encrypting File System, which encrypts files stored on the computer's hard drive so they cannot be read by another user, even with physical access to the storage medium
support for iSCSI protocol
centralized administration features, including Group Policies, Automatic Software Installation and Maintenance, Roaming User Profiles, and Remote Installation Service (RIS)
Symmetric multiprocessing, allowing the PC to divide work between multiple processors (CPUs)
(Windows XP Home Edition does, however, support the Hyper-threading functionality present on some Intel microprocessors. Although it has been reported to work on some dual-core microprocessors available from both AMD and Intel, Microsoft has recommended upgrading to Professional Edition for improved stability and compatibility.)
it sounds like xp pro works better on multiprocessor machines, i don't know how that applies to the imac core duo maybe someone here can clear that up. personally id go with xp pro just in case.