1. It's very reliable, and very capable of rendering over long periods of time. I have been recently playing with
echelon to encode some stuff using the batch feature, as an example, and ran it for around 12 hours straight, with no problems. It rarely kicked the fans in full blast (which surprised me). You'll find the machine is quite useful when your rendering, creating DVDs, etc. because your still able to do normal work while it does all of that in the background.
2. Like I said, it rarely gets loud/runs the fans full-blast.
3. As was said, 4 memory slots is a pain, but 1GB sticks will do just fine for a long time. No other limitations.
4. Hard to call, depends on what you need to do and if your having multiple monitors hooked up. If you have two monitors, go with the ATI for sure, for the extra memory.
5. Incredibly easy to access the insides.
6. Sounds like you got a bad PB - mine has been incredibly reliable. Regardless, you'll enjoy the G5, and it has many years of usefulness to come.