A lot depends on the application and usage that the systems are seeing. Java applications, and MS Office applications that run in Citrix are not always the best candidates unless you want to severely reduce the VM per CPU count. Hardware keys or dongles also are not good candidates either. From my assessments, we are talking about 80% of systems are good candidates but non-technical factors such as political wrangling ("I'd never put MY mission critical app in a VM") and lack of knowledge of what VMware is ("VM-what?")reduce that to below 50%. Another thing is application vendor support which was poor a couple of years ago. That is all changing though as VMware becomes more popular.
We are about to go through another assessment and since this time the servers in question are all older out of warranty hardware, most if not all, are good candidates with average CPU speeds of PII450 and 1Gb of RAM and a lot of NT 4. Putting these in a VM can only improve performance.