The server should have a minimum configuration of a dual core processor with hyper-threading enabled and with a clock speed of at least 2.33 GHz. A faster clock speed and more cores will provide better performance. A minimum of 4 Gigabytes of RAM and 4 virtual cores is adequate unless you have many H.323 transcoding sessions.
For calls out to an H.323 device, the load will be on the server you are connected with Vibe.
For calls in from an H.323 device, the load will be on the gateway the call is connected to.
Roughly 2 cores for H.323 (transcoder) as transcoding is very compute intesive. A 8 Cores server would be able support 4-5 H.3232 connections.
Server Operating System Specification