The answer to this is really "it depends." I'd say if you took a poll you would see everything from old multi-cpu single core machines with 4-8GB of ram all the way up to 4 way nehalem machines with 256GB of ram. As far VMs per host ... that really depends on the host and how many machines it can handle. I've seen everything from and ESXi box running 1 vm (to get around a hardware compatability issue) all the way up to monster servers hosting 50-60 VMs each.
EDIT: in response to the question:
I've seen 50-60 workstation images running on a Dell M600 Blade server with 24GB of ram and the E5506 Proccessor (They were bought before the Hyper threaded models came out)
We also have about 40 Servers running on the same type of host with 48GB of RAM.
My friend has a server (don't know the model) at his office that is a four way opteron systems with 196GB of ram. Not sure how many VMs they are running off it but i'd bet it gets up to the 50-60 range