If you're wanting to do it cheaply, you might look at the RaspberryPi. Probably the cheapest option, and they run XBMC just fine.
No real gaming on a RPi though.
You might consider the Ouya if you're wanting games. They run XBMC about as well as the GBox, and you can play games with it. I've heard there aren't tons of games, and the controller isn't the best. But it's pretty cheap.
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