Another noob question about setting ram?

My MacBook has a Core 2 Duo processor with 4GB RAM. Can I set up my virtual machine for 2 CPUs and 2GB RAM, or do I need to leave 1 CPU for the Mac if I'm going to run both simultaneously (not in bootcamp)? In other words can the two OSes "share" 2 precessors, or does each have to have its own?

Also, if I can set up for 2 CPUs how do I change the VM that's already set up for 1 CPU without reinstalling everything? Can I do this in "Configure?"

 

Yes, this is possible.
Exactly as you say, you can share 2 CPU in Configuration here.
There is no need to reinstall anything, the only thing whioch has to be done is that virtual machine has to be stoppen in order to apply changes.
You can freely test the performance with both, 1 CPU and 2 CPU and see which would be more suitable.
Memory settings can be changed here.