Run Windows inside a virtual machine on Linux for gaming
VMs, by definition, cannot use the actual graphics hardware. It's being used by the host so the guest can't use it too. That's just how it works. You can get better performance by installing the DirectX additions though.
You will need to install DirectX as an add-on for VirtualBox for it to work, but it does work.
To install DirectX, you need to boot the guest into safe mode. Boot your Windows VM into safe mode and go to VBoxGuest additions. Install the Direct3D additions. It has to be done in safe mode or it won't work.
Install the VirtualBox Guest Additions on Windows, and enable "Display → 3D Acceleration" in the virtual machine properties.
You can actually pass through the graphics card to a windows guest, but you have to use something like Xen 4. The biggest caveats to this setup are that you need some patience and knowledge of hardware and linux, and the host OS cannot use the graphics card at the same time as the guests. Generally speaking, you need either scripts to manage moving your card from guest to host and back, or you just pipe it to the guest and access the host through ssh/cygwin X forwarding, etc. Not for the feint of heart, but shouldn't be more than a weekend project for an experienced tech. I have multiple graphics cards passed through to different guests using Xen 4 on top of Fedora, and it really wasn't that bad. Recompiling the kernel to add vt-d support was the thing that took the longest amount of time, but if you're comfortable recompiling your kernel, you should be able to get it to work.
The information out there about requiring FLReset/Function-level reset is old and bogus; neither of my devices have it and they work fine.