I'd say you do a clean install on a second partition. VMWare won't let you experience all the new features tha Windows 7 gives you.
If you want to try VMWARE, you can install Windows 7 and install VMWARE too to see how it works, either in Windows 7 or XP. From there, you can set up a new installation of Windows7, Vista, XP or whatever you want.
VMWARE or Virtual PC are mostly for testing purposes, or for safety reasons, or a backup solution. E.g. I have 3-4 OSes running on VMWARE, and I open each one whenever I want to find a feature how it works so I can explain it to a customer. But that's it. You won't have graphics acceleration, aero theme etc in VMWARE or Virtual PC.
If you have the ability to install on a new partition , do so.