Keep in mind that VMware is designed to consolidate servers, so it is excellent for taking a number of systems that are under utilized and placing them on a single piece of hardware, and then managing resources so that the VMs can "share" those resources.
If you have an application that is extremely intesive, it may not be a good candidate for virtualization. Ken makes a very good point about adding additional CPUs. Also, in general, you want to have vSMP VMs running on seperate machines from single-core VMs. However, it sounds like you are not running multiple ESX servers at this point anyways.
