They should be able to give you some better starting specs to work with if they can so easily drop from 8 CPU core to 4. Unless you expect a heavy CPU load generally you should start with 1 vCPU and increase from that only if you need to.
But in regards to your question about using up 1/2 of the server. When ESX has to schedule a CPU cycle for a VM, it will find an available core from among the 8 in your host. So a VM could move around if a server is busy. With ESX 2 and up to 3.5 U 3, if you had a 4 vCPU VM ESX would have to find 4 core that would be free pretty much at the same time. So you could end up with performance issues if you have a number of multi vCPU VMs running as they are harder to schedule when the server is under a higher CPU load. With ESX 3.5 U4 and continuing in ESX 4 improvements have been made to CPU scheduler to relax how close the 4 vCPUs would have to execute on physical cores enabling easier scheduling Also, with the improvement if a guest doesn't require all of it's CPU cores then ESXi won't have to schedule any physical CPU cores for idle vCPUs.
What happens if your VMs has a low CPU usage or is idle? ESX is free to use the CPU cores for other VMs. So you would not loose 1/2 the capacity of your host if you had a single 4 vCPU VM running. Depending on the load of your VMs it's realistic to get 5 - 8 vCPUs per physical CPU core.
Dave
VMware Communities User Moderator
New book in town - vSphere Quick Start Guide -http://www.yellow-bricks.com/2009/08/12/new-book-in-town-vsphere-quick-start-guide/.
Do you have a system or PCI card working with VMDirectPath? Submit your specs to the Unofficial VMDirectPath HCL - http://www.vm-help.com/forum/viewforum.php?f=21.
