The only AMD mini-ITX boards I can find are FM2 , so the most powerful processor you could use would be the A10 5800K
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007625+600009028&QksAutoSuggestion=&ShowDeactivatedMark=False&Configurator=&IsNodeId=1&Subcategory=22&description=&hisInDesc=&Ntk=&CFG=&SpeTabStoreType=&AdvancedSearch=1&srchInDesc=
and the processor
http://www.newegg.com/Product/Product.aspx?Item=N82E16819113280&Tpk=a10%205800k
Add a 2x4 gig kit of 1866 Mhz memory and you have the basis for a midrange gamer even without a graphics card .
Performance like
http://www.tomshardware.com/reviews/memory-bandwidth-scaling-trinity,3419.html
or just add a graphics card .A 7850 or 7870 , or GTX 660