I would not do as Chris says and put the graphics card hot. Do the PRAM battery first. Reseating the graphics card is a good idea but not hot. Your hierarchy chart appears correct. If the former user was getting a question mark, and he probably was, because he knew about it, that's because it could not find a system. Either corrupted system or, more likely a bad hard drive. After you get some power, boot up from the Optical drive with a system disk for this machine or a retail system 10.5 or less.