Can I plug in devices to this port and have them be viewed on my imac screen? Like if I had a dreamcast vga box and got the mini dvi vga cord.. Can I do that?
Nope. The iMac has video outputs, not inputs. You could buy a cheap DV bridge, or a TV tuner that has inputs (you wouldn't really want to use these for gaming on the XBOX). Your best bet is an independent LCD monitor with S-video or composite inputs, or if your XBOX has DVI output, you could use that.