I use VMWare Fusion Professional for software testing. Virtual machines make regression testing and replay of problems wonderfully easy.
I run a copy of VMWare Fusion on my older Mac Pro and it performs well. The professional edition allows for linked virtual machines – linked machines can share common content to save on disk space. Machines can quickly require ~20GB if no content is shared.
Snow Leopard Server
Be aware that Mac OS X 10.6 Server is required for a virtual machine. The standard client edition is not permitted to run within a virtual environment.
OS X 10.7 and later can all be legally run within a virtual environment, so long as the underlying hardware is an Apple computer.
You are right to opt for as many resources as possible for your Mac. Running virtual machines can push your Mac. Try not to require needing multiple virtual machines running at once.
Graphical applications or those needing OpenGL support are not yet well supported by either VMWare or Parallels. OS X copes but falls back to software rendering; this may not best match your customers' environment.