How many cores should I assign for the VM?

Currently I'm trying to run windows on my mac but during the installation the configuration assistant asks me to assign a number of cores to the VM, along with some RAM memory and Vid memory. How should I set this? What does it mean if I use all cores or all memory? would that affect the performance of other apple apps or just "let" the VM use the resources (which I don't see why it could be bad).

I need to run windows primarily to code and test software.
And maybe run a couple windows-only games but I can live without that...

 

i would start with 2 cores and 3 or 4 gigs of ram and 256MB video memory and go from there. You can also add more later easily via the configuration window and make sure use adaptive hypervisor is check and it will allow more resources to whatever system is in view.