What is the purpose of the virtual computers?
Running another server?
Basicly your options:
Virtual PC - Microsoft - Free, works well for doing testing, very clean quick setup, nice interface. No USB support
Virtual Server - Microsoft - Free, never used it, because I could never it the configuration right.
Virtual Server - VMWare, - Free, using it now, will allow 64bit guest on 32bit host os, includes USB support, managed though a web interface which I find kinda slow and clunky but works ok. Allows you to take 1 snapshot which you can revert back to. This is good if you are testing something and need to role back your OS if it doesn't work or for other tests. This is a good starting point to try stuff out.
Virtual Workstation - VMWare - Not free but only a few hundred $$$, ideal if you are doing a lot of testing, nice tabbed interface (last time I checked) allows multiple snap shots which is awsome for trying out different configurations for testing.
ESX, ESXi & Hyper V are designed for production level servers and basicly don't have a host & guest OS, all the OS's run within the hypervisor. Of this ESXi is free if you wanted to try one.
And you can never have too much memory, you need as much for each machine as you would normally have for that machine if it wasn't a VM.
IE if you have 3 VM's and you want each one to have 1 gig of RAM, you should have a computer with 4 gigs since you need 1 + 1 + 1 + something for the host too.