Greetings!
I built my system with much the same intent as you (to get Microsoft out of the equation), and thus far I have been able to get all of my games to run flawlessly in a WindowsXP or Windows7 VM environment. Now, for the catch... I am not a big gamer, and I tend to play older games (the newest game I have is Doom 3 - circa 2003 - which plays great at high resolutions on Win XP with all the bells and whistles turned on). Other games I play are "Unreal Tournament," "Half-life," and several others of that vintage, including many 2D games. All of the 3D games work fine so far, keeping my expectations in check (the audio is not perfect, etc). As for newer games that require the latest video GPU developments, a VM is probably going to be somewhat of a disappointment (I have heard that there is a way to pass-through directly to the video card hardware on the host, but I don't know much about that). In my case, the host hardware is so overkill, that the requirements of the games I play in the VM are fine. I keep wondering when someone will realize that virtual gaming is not some niche, and release a VM manager optimized for just that. Someday...