VMs are a great idea, as long as you duplicate the farm as closely as
possible. Are you looking for more specifics? That might depend on what
you're going to be testing.

I wasn't planning on duplicating the entire farm, but rather duplicating by
site collection as necessary. I am mainly going to be using the Dev/QA
environment for testing new features--whether those features be out of the
box SharePoint items or third-party add-on's.