It's certainly possible to build a power-house mini-ITX build.
Example: GIGABYTE GA-H55N-USB3 LGA 1156 H55 HDMI USB 3.0 Mini ITX MB £85
and Core i7 860 Lynnfield, 2.8GHz £212
So the limiting factor, size wise, is what video card you want for your gaming. For example, a GTX 460 or HD 5850.
With a card that size the case that will fit a card that size will also fit a micro-ATX motherboard.
IMO the only reason to go mini-ITX is if you can find a half size video card to fit inside a really small case that also has a PSU powerful enough for the video card. PowerColor Low-Profile HD 5700 Series Cards Spotted
Have you figured out what need for a video card? That will likely be the biggest, and most power hungry part and you can build your system around that.