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Great question (and potential five page answer). I see this all the time, and the only way to answer the question is with math and real numbers. It is impossible to say any server will be 'fine' as VM. Sure it will run, but will it perform its tasks as it does today without the end user noticing (and that is the key, does the end user see a difference?).
Of course most people don't want to sit next to some users after a migration and find out, but instead want a way to determine candidate potential ahead time. We do this (and have done it thousands of times) by rationalizing what is used today to real numbers.
Let's take processor usage: If an existing machine has a dual processor 3 Hz server (total of 6GHz), but during the day only averages about 10% utilization, then the machines needs 600 Mhz. Just fine for a single processor VM. We have even ranked tier 1 candidates up into the 1.5GHz range.
If the application is running takes advantage of SMP, then maybe a dual processor VM would work for it. From a memory perspective, it is pretty much a wash. If your machine uses 1.5GB now, it will use 1.5GB as VM. Of course, ESX-based VMs can now go at 16GB of memory, but who would want to at today's memory costs for 8GB DIMMs.
For disk and network there is a certain penalty to pay. On the disk side it is about 10-20%. But of course most Wintel servers never average above 10 or 20 IOps a sec, or ever get near the SCSI card's throughput capabilities. On the network side the penalty is a bit higher. Then again, a recent client I worked with who had 1000+ servers being surveyed only had about 50 or so that used more than 1MB on average… does every server really need dual redundant 1GB NICs all to themselves?
Show your coworkers the numbers. Work from the facts and your argument will be more persuasive.
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