I installed more RAM in my iMac G5 last night. It was an interesting, and fairly painless, experience. I put down a blanket and placed the machine face down, and found the three captive screws on the base of the computer/display. The screws came out smoothly and (nicely) remained captive so that I didn't have to keep track of them. Then I pried the entire back of the machine off to behold the beast inside. The G5 chip and its cooling mechanism do dominate a fair amount of the machine. The RAM slots were easy to find and the new DIMM went into place without problems. Putting the base back on and plugging the machine back in took just a couple of minutes, and reboots on these machines are lightning quick. I was impressed with the whole experience, which was basically on par with my upgrade experience on my old iMac DV if not better (it's cool seeing all of the parts in the machine).
According to Apple, the iMac G5 is the most user serviceable iMac ever, and the list of what can be replaced by hand is pretty impressive considering how closed off previous generations were. I wonder if in the future, Apple or other dealers will offer replacement mid plane assembly boards with faster G5 chips or improved video chips and ram? It doesn't seem terribly likely, but it is nice to know that I have a few options to hopefully sustain the life of this machine should I need to.
Then again - except for its video card, I've never had any real complaint about my venerable iMac DV SE 400. Over the last 4.5 years, the only problem has been its front power switch, and that itself was rarely an issue.