I don't really care if my 933MHz Quicksilver is slower than a Mac Mini. I just won a Sapphire9250 on Ebay for $14, and that will give me four times the video memory of the last Mac Mini. I've also purchased a Nec based USB2.0 card and a Sounblaster Live for the system (neither of which could be added to a Mini).
Besides, even if you don't factor in the extra expansion capability, Powermacs just look cooler. And, right now, they're actually cheaper than Mac Minis.