As well as our own OCS sizing tool that you can download from here HP have released their own utility:
New Unified Communications tool available
Check out the HP Architecture Planning Tool for Microsoft Office Communication Server 2007
Along with Microsoft Exchange Server 2007, Microsoft Office Communication Server (OCS) 2007 is a core component of Microsoft's Unified Communications strategy-providing instant messaging and real-time presence information, audio/video conferencing, and Enterprise Voice over IP (VoIP). The HP architecture planning tool for Microsoft OCS 2007 offers a quick, easy way to generate HP server and storage hardware recommendations based on the features and functionality required for a specific OCS 2007 topology.
Download the tool on HP ActiveAnswers now.
Note: The first mode of the tool provides a simpler user interface on a single page and is designed to generate an example OCS configuration for a single site. The second mode provides a more in depth user interface and supports a multi-site OCS topology and more granular user input options to further refine the specifics of the OCS configuration. Use this to form the framework for a discussion around the cost and server/storage infrastructure requirements needed to deploy Microsoft OCS 2007 on HP technology