also, it's important to remeber that 533 MHz PowerMacs and under were the old 4 stage pipeline G4, before they switched to 7 step.
a dual 533 G4 will absolutely blow a 1 GHz G4 out of the water in basically every aspect of use, probably better performance than the low end mac mini as well. the dual 500 in that auction, is, despite the illogical reasoning of the seller, probably just about as good as a single 1 GHz G4.