Q: Why G5 PCIe?
A: Because it has 4 USB sockets instead of 3 in G5 AGP!!!
You didn't look well, my friend. The fourth USB in G5 AGP is there, but well hidden (amongst AGP Pro extension pins). That's why any ADC monitor will be also a USB hub too.
(not that I reccomend using ADC - I have a strong belief that an ADC montitor blew one of the power lines on my Mac Radeon 9800 effectively trashing it)
Just why I mentioned "4 USB sockets", not pins or whatever