So I just delivered a webcast this past friday on Capacity planning for OCS 2007; the link can be found at this location
http://msevents.microsoft.com/CUI/EventDetail.aspx?EventID=1032372032&Culture=en-US. The webcast includes information in regards to the different topologies that are available with OCS such the following:
1) Standard Edition Topology
2) Collocated Enterprise Topology
3) Expanded Enterprise Topology
And then I also address the two topologies of the edge such as the consolidated and single-site approaches. The other cool things are I go into details with numbers of how to determine how many IM can a Front-End handle, how many web or AV conferences can a server handle as well.
To know more you have to see the webcast and from what I understand, is that they will also be available as podcast soon as well.