I have the 13" rMBP and run a Windows 8 VM for my work stuff all the time. I give it 2 GB RAM and it runs shockingly well. My problem is the disk space. I only have the 128 GB model so I can't fit multiple VMs easily. I have a 64 GB USB 3 stick that I use as swing space, but I should probably just get an external drive with more space.
I can run a second VM with another 2GB of RAM and it works too. Just not so great on the battery life. But I only run two VMs at once while I'm building out a new Windows VM for use.
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Also, I'm sure they will have 16 GB in the 13" retina at some point. However, by that time comes, you'll probably want 32 GB. 🙂