Hello, and sorry for the lengthy post (I'm trying to be as thorough as possible)! My buddy has this early 2008 iMac that's...

If it's the backlight, then the problem is most likely in the circuits and it's time to bring out the multimeter and the understanding of circuits and their components. You could start out first by eyeballing. Check for bloated, popped or otherwise disfigured capacitors (google images may help you know what to look for). Although replacing a faulty one is often only half the problem as a popped one is often just a symptom rather than the cause. You need a soldering iron to replace these. Now for the fuse, those are normally marked with an upper case F followed by a low number. Also they are normally marked further with a rectangle border around the component. Lastly, they are located very early on the board where the power comes in as their purpose is to break to protect the rest of the components in the circuit in case of a bad voltage. Now if the fuse and capacitors are healthy I recommend consulting an expert as the next steps require measuring and knowing what voltages should be where (advanced repair...

I would reopen the phone and check if all the ribbon cables connecting the display to the phone are correctly seated. You can use the beginning (up to step 11) of this guide to verify what goes where and how it should go there: iPhone 3G バッテリー修理 Also, while you're in there, check for anything that may have bent or come loose the last time you visited. Such incidents can cause the phone to short out and refuse to boot. Lastly, be very careful when you're working on the phone since the battery is still connected and possibly charged. Preferably remove the ribbon cables using your fingers or something plastic. No poking around with metal! :)

The flashing question mark folder means that it's not detecting any bootable device. You could try finding a dmg of MacOS X Leopard (not Snow Leopard as that's Intel Only) and burning it to a Dual Layer DVD disc. Then you should be able to boot and install from that by holding down the option key while starting up (gives you a list of available bootable drives). I had two PowerMac G5s a while back and they both were pretty stubborn when it came to bootable USB, but bootable CDs and DVDs were no problem. Regarding the Invalid Memory Access error, have you tried swapping out the RAM? It may be that you have a bad stick of RAM. Depending on the model you have, you've either got DDR or DDR2 RAM which are fairly common and cheap when bought second hand (there's at least a pair rattling around in every man-drawer out there). Again, depending on your model, you could install up to 16GB of ram! Also, If you're feeling particularly adventurous, you could install a flavour of Linux instead of OS X. Debian and Ubuntu...