Reducing the amount of RAM the virtual machine takes may help the OS X side of the equation. Running two VMs I found my Mac sluggish. My first generation MPB is full with only 2GB. I reviewed in each VM how much RAM was being used by the applications I normally run and found I was giving the VMs more ram than they required. I reduced the VM RAM settings and now the Mac isn't in as much trouble because there is more remaining for my OS X applications to use.

I also prefer creating the full size disk image size I think I'll need. Windows on the VM has given me warnings that it is out of Windows VM disk space and the time it takes to append more space to the virtual drive is very time consuming which stalls my work. In this case I'm running BI server software and Oracle. The draw back to creating a fixed size disk is that I could run out of space for a virtual machine. That is fixable though.