This probably depends on what you will be using your Windows VM for. If you are going to run resource intensive apps, you will probably want to use as much as possible. There are quite a number of tips for optimisation (will have to go look for some links, but google will also help you out)
In my experience, the following helped a great deal:
RAM will be dependant on which resources you need more, Win or Mac. Try not to go below about 1.3GB for the Mac, else it will be very sluggish when your VM runs.
I think 2 CPU's are non-negotiable
Hypervisor should be on
Fixed size hard drive is recommended
Assign as little graphics memory as needed, and also turn off the nice aero themes etc if you dont need it. These chow performance
In the end, you will have to play around to see what works best for you. There is a rather nice article
here where they actually tested performance with different settings, should give you a nice indication of where to look. There are many more resources online, but the tweaks are about 90% the same. The rest is up to your own personal preferences/needs