Capability wanted Servers to Install
Just IM, no new meeting capabilities, but still supports Sametime classic meetings IBM Lotus Domino
IBM Lotus Sametime Standard V8.5 Community Server
IBM Lotus Sametime Standard V8.5 Connect Clients
Optional LDAP directory source
Ability to schedule meetings, and restrict who can access the meeting based upon their user name. (Sametime classic meetings) IBM Lotus Domino
IBM Lotus Sametime Standard V8.5 Community Server
IBM Lotus Sametime Standard V8.5 Connect Clients

Optional LDAP directory source

New Sametime 8.5 Meeting Capabilities without Audio and Video. IBM Lotus Domino
IBM Lotus Sametime Standard V8.5 Community Server
IBM Lotus Sametime Standard V8.5 System Console Server
IBM Lotus Sametime Standard V8.5 Meeting Server
IBM Lotus Sametime Standard V8.5 Connect Clients
LDAP directory source
New Sametime 8.5 Meeting Capabilities with Audio and Video. IBM Lotus Domino
IBM Lotus Sametime Standard V8.5 Community Server
IBM Lotus Sametime Standard V8.5 System Console Server
IBM Lotus Sametime Standard V8.5 Meeting Server
IBM Lotus Sametime Standard V8.5 Media Manager
IBM Lotus Sametime Standard V8.5 Connect Clients
LDAP directory source
All Sametime 8.5 capabilities, and support for mobile devices like iPhone, and zero download web client (AJAX) IBM Lotus Domino

IBM Lotus Sametime Standard V8.5 Community Server
IBM Lotus Sametime Standard V8.5 System Console Server
IBM Lotus Sametime Standard V8.5 Meeting Server
IBM Lotus Sametime Standard V8.5 Media Manager
IBM Lotus Sametime Standard V8.5 Proxy Server
IBM Lotus Sametime Standard V8.5 Connect Clients
LDAP directory source

Sametime 8.0.x IM capabilities running on Domino 8.5, and support for zero download web client (AJAX) and mobile devices like iPhone IBM Lotus Domino

IBM Lotus Sametime Standard V8.5 Community Server
IBM Lotus Sametime Standard V8.5 System Console Server
IBM Lotus Sametime Standard V8.5 Proxy Server
IBM Lotus Sametime Standard V8.5 Connect Clients
LDAP directory source

The number of servers depends upon the options you want. The colour indicates minimum servers you will require, in reality many of these servers under load will be placed on additional machines.

Minimum 1 Machine
Minimum 2 Machines

Original Source: My thread on the IBM forums which was responded to by Carl Tyler who also blogged it here