What is the minimum hardware I should get for a low-power media set-top box, using mini-itx?
This is a very hard question. There are 3 main factors here. Your CPU power, how much video acceleration hardware is available and what kind of video capture card you have (if any). The best thing to do is searching the freevo-user mailing list. Freevo users are quite fond of small/silent machines and much of the generally available “minihardware” has been tried & tested. You probably will find an answer there.