here is the kicker though, any full ITX builds especially for gaming will weigh in around 10kg even if it fits in a backpack and its still not what i consider to be comfortable to carry.
things like an ATX PSU , heatsinks and GPU add up to around 5kg not to mention the case itself and HDD.
if you really need to carry one then you will need to do a lot of chopping and sacrifices.
IMO a monster laptop will offer more performance in a more easy to carry package then a desktop after the chop.
anyway what i use for my backpack rig is a ISK100 box with a A10 5800k APU. its slightly larger then a mac mini but more capable and fairly light.
i carry that ISK build along with my SG08 and CM elite 120 around all the time for my gaming virtualization project