Thomth, I have some experience with modding the iMac G5 and may be able to help. For the most part I have converted them to external monitors (using parts from a 20" Cinema Display), however I briefly thought about putting an ECX (3.5" core 2 duo motherboard in the G5). In terms of the depth of the iMac G5, it varies, the iMac G5's design evolved considerably over time. The first generation iMac G5 is about 2" deep and the LCD attaches to the front panel of the case. This will not fit a standard mini itx, but actually has a decent amount of room. Just to let you know the ECX board (Keex-4020) did fit in the area below the LCD, however, I do not remember if it would have fit behind the LCD itself.
