actually, it seems very easy to add stuff. they say that the side panel opens up and you just remove the old hardware and put in new hardware, and when its clicked in, your done and its ready to use.

"Up to 500GB of internal storage
The Power Mac G5 has two Serial ATA hard drive bays for up to 500GB of fast internal mass storage — ideal for video, audio and high-resolution graphics. Built-in software RAID allows you to stripe the two drives for increased performance or mirror them for high reliability. And it’s easy to add storage as your needs grow. Single-drive systems come with the cabling to connect a second hard drive, and the easy-to-open side panel lets you insert a new drive yourself. Want massive system storage? Just use a Fibre Channel PCI card to connect to Xserve RAID, Apple’s high-performance storage system."

but then again, thats from the point of view of apple. ive never actually owned one. I have used one though, and i like the fact that everything just seems much easier to do on a mac than on a pc.