I would second VMWares capacity planner. It is installed a physical box, can be a desktop even. It needs access to all the networks and within a few days up to a month it will tell you what your machines are running and what type of servers, CPU, memory and bandwidth you'll need. It is relatively cheap.
I worked a place that had about 50 physical servers in 2007. We virtualized all 50 on to 3 Dell 2950s with 32gb of RAM and an EMC fiber channel SAN. Right now my environment is about 75 servers and desktops on 3 Dell R610's with 64gb of memory each, dual 6 core processors on each and a Dell Equallogic SAN.
If that gives you any idea of what you'll need, really it'll be based on your environment and you're needs.
Cheers
Rob
The answer is always "PEBKAC!