So what about a Mac tower? You can add video cards and hard drives to your hearts content. You are only as limited as your case. The G5 tower is plenty large enough to make modifications simple. How many hard drives can you fit in your tower versus a G5 tower? I'd imagine they're just about even.
http://macmod.com/content/view/171/2/ shows the G5 tower with 10 hard drives, same as you mentioned (myth busted beyond repair). Four hard drives in front of the main processor fans, two above the opticaldrive, two in the hard drive bay, and two in the PCI slot area.
It is a common myth that Macs cannot be modified. No matter how common a myth is, that still doesn't change the fact that it's still a myth.