Need to know if it is an iMac or Mac Pro? If you are talking about an iMac you will have to take it apart. How you accomplish this depends on the year it was made. YouTube has plenty of videos showing how to take an iMac, Mac Mini, or Mac Pro apart.
The iMac G3 memory slots take PC100 SDRAM.
Just connect a digital camera to the iMac and turn the camera on. iPhoto will open and import the photos or videos. Be patient it may take awhile the first time you do it.
Take a look at your system motherboard to see if it has an AGP or PCI Express card slot (it will be the topmost expansion slot on the motherboard--your monitor should be connected to the card currently sitting there). This will tell you which type of video card is available to you.
Pack it securely with lots of bubble wrap and hope for the best.
Plug your camera or card reader into your mac via a usb cable. Then open up iphoto and click the import all photos button. It may take a while to import if the file sizes are larger.
Your motherboard/bios tells the video card to turn itself on/off. This particular model does it a little later than your old board did. If you can reboot and see more of your boot up sequence than on "power up", then this is the case. The the monitor would already had received its power on command and be ready to turn on than from cold state.
Command > Shift 4 for a screenshot Command > Shift 3 for a predetermined screenshot. To take a photo with the iMac built in camera open Photo Booth in Applications.
All the software you need comes preinstalled on an iMac.
iMACs are made without easily-accessible compartments for most components, particularly memory, which means the only way to add RAM is to take the iMAC to an authorized Apple dealer and have them add it. If they have what is needed to make the addition, it is likely to be a pricey one, so get a quote first in case it is less costly to just but a newer, higher RAM version of the iMAC.
This seems to be more of a technical problem. You should take your iMac into the nearest Apple Authorised Service Centre for help.
You should not bother to check any power supply in the motherboard. You need to take the motherboard to service centre if faulty.