Tim,

In my production datacenter I am running HP DL385 G7 Series Servers with 12 Core Opterons, 192Gb RAM, ESXi 4.1 and average 30-40 VM's with power to spare. It really comes down to density, how may VM's can you get per core and what will it cost. The DL385 is a 2 socket design which can provide you cost savings on licensing over a 4 socket design. The 12 core processors provide enough horsepower for good density numbers. My host CPU averages 8%-11% during off hours and 11%-22% during peak hours. We performed density testing on several models before we settled on our spec. For testing we created a load test template which we used for rapid deployment on each platform using W2003R2 with 2CPU, 2GB RAM and a script which loaded the CPU's and compensated for differences in physical processor speeds. One day I will see how many VM's it takes to get my host CPU utilization up to 80% range!

Mark