1 vs 2 processors in HP DL380P for Virtualization

2 CPUs period

The memory on the HP DL servers is balanced accordingly to CPU so you will only be limited in the number of memory you can place in the server.  If you reach a certain limit you would not only need to buy memory but the CPU kit as well which will cost you more in the long run as it may no longer be available readily

NOTE:
If only one processor is installed, only 12 DIMM slots are available.  Memory is usually the chokepoint and is cheap, get the 2 CPUs now and you don't have to worry later

Thank so much Rick! The buget allows for the E5-2609v2 2.5GHz processor which as you said, runs the memory bus at 1333 MHz. If I were to upgrade to a processor with a 1600 MHz memory bus, like the 2.6 GHZ E5-2630v2, do you think it would make a big performance diffence for our SQL database app?

Purchase the server with the higher clock speeds.

We've just purchased servers with 10 Core E5-2697v2 3.0GHz, rather than the 12 Core, which is clocked lower!

I have read article online stating that you should not allocate more vCPU to a VM than you have CPU Cores in the Host. Otherwise the VM would need to span the memory bank of another CPU... I believe it has to do with NUMA..

Just some food for thought

Always start with 1 CPU and if you need more add more but you can slow down VMs by adding too many vCPUs. If you can keep the amount of vCPUs on ALL your  VMs lower than your QTY logical processors you will be better off.