These posts about vmware aren't one size fits all, although they seem to be roughly correct for desktop usage (which is what HMTK asked about). For very small implementations, such as 1-2 VMs (or more), which are mainly used for testing other OSes, or running apps not available on the host, then RAM is certainly the bottleneck on a desktop or laptop.
For larger implementations, where there are 4-8 VMs running a workload on a larger system, both RAM and CPU, as well as disk I/O can constrain the system. Dual Opterons have had about 40% better performance than Xeons for the apps I've tested (not a wide variety though). These are somewhat CPU intensive apps, and they are active at the same time.
On very large workloads, with VMWARE ESX, CPU is almost always the bottleneck, although driver and architectural limitations have caused us to change configurations or reduce the number of VMs as well. From my testing, we expect to run 40-50 VMs per 8-way x445 (with 32-64GB of RAM). Under the load tests, CPU peaks far before any other resource, and other organizations have had similar results. 30-50 VMs seems to be standard.
Also, VMWare recommends allocation about 64MB more RAM for a VM than you would put into a physical machine when creating it.