I can honestly say we are 100% virtual. i have not found anything that can not run as good in vmware if not better, if allocated the correct Disk I/O, and Resource Reservations. I'm not saying that EVERYTHING can be virtualized. i'm just saying as a fairly large Urology Practice with over 70 VM Servers, we have had "ZERO" performance issues related to Virtualization. ( Well if you ignore the insanity that was trying to run all that on NFS data stores, once i switched to iSCSI all was good).
currently I have 8 hosts, 2 socket 4 cores w/HT (16 logical proc per host) with 96GB ram ea, allot of my hosts are running at that 70% memory mark, i was planning on doubling RAM in the next year or two once 8GB dims became a little more reasonable. our CPU utilization is nill.
Honestly i'll have 1gig iSCSI (currently 4 nics dedicated to iscsi per host) congestion long before i'll have CPU congestion. i was looking at upgrading to 10gige.
frankly i underestimated how dense we could go per host. i am looking at shutting down 2 hosts now and not refreshing that hardware down the road.