The core assignment is usually dynamic. You only set the maximum amount of cores that the VM is allowed to use at any load level. If you set 2 of 4 than even when OSX idles the VM takes only 2 cores under full load. Or if you have multiple VMs running you can limit each to 1 core or something.
Via msdnaa you can get a couple Windows licenses for free. If you are a computer science, MIS student or anything similar chances are good your university offers msdnaa accounts for you. You need to be a student of course.
Parallels is never free unless you hack it. I would also recommend VMWare Fusion as it is more stable and trouble free in my experience. For gaming and 3D acc. use native Windows via bootcamp.
VirtualBox is a free VM but it is usually more hassle. I would try it though if is enough for you there is no reason to pay for VMware or Parallels.