i3 3220+Gigabyte GA-B75M-D3H is fine.Fx8120 would fall behind in majority of games,too low clocks and too low light threaded performance.
FM2 A55 chipset mobos don't have SATA III and usb3.0 ports.Also for most of the games at present i3 3220 is better than a10 5800k.
Check out the recent games like far cry3,hitman absolution,i3 3220 is better than a10 5800k.Even in crysis 3 ,i3 3220 performs better in some scenes and in some a10 5800k performs better.A10 5800k with an A75/A85x chipset(which have SATAIII and usb3.0)mobo would cost ~3k more than i3 3220+B75 mobo,the money saved can be spend on a better GPU.