That's why I mentioned PCI-Passthrough. Then the guest can directly drive the graphics card, with it's native drivers.
But you and johncolanduoni both say that it's nontrivial. However, blibble says that it's easy with current ATI consumer hardware.
I know a few gamers who would like to relegate Windows to a locked-down VM ;)
Well, it was a while that I was looking at it. Combine that with my opinions about gamers' hardware and system software knowledge (not very flattering, outside of knowing which brands and models are "best"), that's how I came to the conclusion, that it might be difficult for them.
But do not take that as discouragement, your friends definitely should try it on their own.
So then you run Linux VMs in a Windows host. In my experience, VMware is a better choice on Windows hosts. But it's been a few years, and maybe VirtualBox is doing better on Windows.