[CentOS] Setting up a Virtual Machine

I have CentOS 5.7. How would I go about setting up a virtual machine so 
as to run another Linux distro on top of it? Specifically Scientific 
Linux or Vinux, a special distro for blind users. I will be using the 
latter to test my BrailelBlaster software with gtk and the Orca 
screenreader. Will I have to assign memory to the VM permanently?

Thanks,
John

-- 
John J. Boyer; President, Chief Software Developer
Abilitiessoft, Inc.
http://www.abilitiessoft.com
Madison, Wisconsin USA
Developing software for people with disabilities