The difference is that if you use Boot Camp Assistant you will be booting into either OSx or Windows separately. When booted into Windows it will be like you are using a Windows PC and Windows will use all your computer's resources. To use OSx again you will have to shut down your computer and then boot into OSx which will then use all your computer's resources. This is the method I use. More information about Boot Camp is located here: http://www.apple.com/support/bootcamp/
If you install Windows using a virtual machine then you boot into OSx and then start up Windows. So both OSx and Windows are running at the same time sharing your computer's resources. If you are running resource intensive programs then this is not the preferred method since resources are shared. But it is easier to setup Windows with a virtual machine. VirtualBox is free. For more information about using a virtual machine visit the Parallels, Fusion, or VirtualBox website.
MS Project and Visio are not resource intensive programs. Action games would be resource intensive.
Regardless of method selected, you will need to buy Windows.