I won't go into detail wether or not I think the AmigaOne will be upgraded to Artica P. Consider though, that PCI-X is only 133MHz point to point (i.e only the bridge and a PCI device will appear on one bus).
If you want two or three devices on a single bus, the speed drops to 100MHz and four or more will drop to 66MHz.
I admit even the slowest PCI-X clock (66MHz) is a far cry from the 33MHz 32-bit PCI system that is prevalent at the moment. Don't forget that the bus will opperate at the speed of the slowest device, so don't go sticking any legacy hardware in there or you'll end up with PCI32-33 or PCI64-33.
The advantages of DDR speaks for iteself, and the efforts are immediately rewarded, it's no easy ride getting 333MHz DDR tracked correctly though!
Neil Thomas, AKA MiniBobF