>why the Q66 would not be the most suitable CPU
It depends on what is going to run in your virtual servers - if they were given heavy compute tasks that could not be spread out across processors, you may end up having one or two processors pegged at 100% utilization for a while, whereas a faster dual core may finish sooner and free up the resources. The quad core is a better general purpose cpu, but faster dual cores can be more suitable for heavy computation bound tasks that are not multithreaded.
The G35 chipset is a good choice for the Core2 Duo processors and the Asus model has been well received.