At my previous job I had a 4 core Intel processor + 4GB RAM. Ubuntu as host and I regularly ran 3 guest Windows instances at once without any slowdowns. I guess it depends on how you do things: the host OS only really ran a terminal emulator + vim. Maybe it would have been different if I had a GUI IDE...
I generally find the quality of multiprocessing in the guest OS is not good; nor is accelerated graphics. Most VMs are ok if all you need is single proc non visual performance, though.
An SSD + 4 cores made the difference for me to finally move my Linux "box" into a VM and not look back. But again, I'm also just doing terminals, vim, and Chrome.
I have an SSD, 8 cores, 12 GB RAM etc., but trying to get processes in the guest OS to scale across multiple cores is very much an uphill battle. VMWare's dual-core emulation was actually worse than single processor for my uses cases.
