"Parentboard" is probably better than "mainboard", but given the choice between "motherboard" and "mainboard", which are the two currently-accepted terms for that particular component, I'm going to go with mainboard.

Edit: isn't the videocard technically a "non-main-board"? I mean nobody called it a "daughterboard" but it kind of technically is one isn't it? And so if "mainboard" implies the existence of "non-main-boards"... that accurately describes its relationship with the video card, which is a baord.