Hyperthreading Enabled (Not active) - Should I turn it on at all?

Environment:

ESX Host - IBM Blade's HS21

Virtual Center 2.0

ESX 3.0.1

Hi,

Some of our application developers are requesting that on a cluster we allow their application to access multiple processors.  (The application is using ASP.NET pipeline processing to address multiple processors.)

I have been told that adding additional processors to a virtual machine on ESX is not always going to solve performance problems as not all applications are able to take advantage of having multiple CPUs.

The developers request is to fulfil the requirement of applications on 8 new Virtual machines, however the cluster the VMs will reside in hosts about 70 other VM Servers whose applications may or may not utilise more than 1 processor.

So, my question is 2 fold:

1) How do I turn on Hyperthreading on my ESX host and is it supported on IBM HS21's?

2) Should I turn it on at all and add 2 cpus to these VMs at all, and could this be detrimental towards other VMs performance?

Thanks for your thoughts on this.

Kind regards,

Dave Cleghorn