Gaming on Virtual PC is not feasible because the emulator does not take advantage of hardware Direct3D support. It only emulates some ancient STB 2D only video card. Because Macs and PCs use the same type of video cards, hardware support would be theoretically possible, but trying to fudge hardware level access that DirectX anticipates would be challenging from a programming standpoint. Furthermore, Microsoft is not positioning, developing, nor testing Virtual PC as a gaming solution.
All that being said, I don't think you really need a PC emulator to play games on a Mac. A surprising amount of games have been ported to the Mac platform: see http://www.insidemacgames.com/