You think a mini-ITX gaming rig is small? Wait ‘til you try a 27" iMac with a 2GB 680MX GPU. I can play Tomb Raider, Bioshock Infinite, SCII, D3, WOW, CoH, Dishonored, Assassin’s Creed 3, DeadSpace 3. . .all on MAXSETTINGS on a PC the size of a MONITOR! It’s amazing. Plug in an XBOX 360 USB controller and you are good to go. Need to take your computer somewhere? Easy peasy. The thing is like 20LB and super thin. And it is ALL IN ONE. Ultimate LAN party machine?
I recommend bootcamping Windows on the iMac though. Drop in your own RAM. And get Macfan to spin up your fan if you are worried about temps. It’s THEBEST way to game.