8gb-16gb
I run 16 VMs on lots of different computers at work -- some iMacs and some cMPs. The iMacs we have are vintage 2008, and I fitted them with 16GB each. I should also say, they all run Win 8.1. If you are going to run Windows, use 8.1 or the new technical preview Win 10. Our experience with Win 7 has not been good -- Win 7 is hugely bloated, takes up a lot of CPU resources and hick ups even our 3,1 MPs outfitted with 32GB. Win 8 and Win 10 on the other hand, I was able to run side by side on a different 3,1 MP with 16GB without any speed issues.
So as the last post said, it depends on your needs. We run VM Ware Fusion because we need a nice commercial product which is solidly supported. Virtual Box is not that.
If you are memory limited, as some of our 3,1 MPs only have 8GB, then make sure the OS you are trying to run has a very compact code. Win 8.1 is precisely that as it can run on cheap tablets sporting 1GB of memory only. If you are going to run something like Win 7, I would suggest at least 8GB, but more likely 16GB. Even then, it will not run very smoothly if you are talking about day to day operations. I would steer away from an OS like that. Trust me, lots of experience with everything from Win XP, to Win 7, then to Win 8, and now finally Win 10.