PCI is an interface...what powers the card is the GPU, the chip itself.
The point you miss is the PCI interface is a bottleneck. And depending on the game it can be a noticeable one. The X1550 still is OK, but considering the new crop of intergrated, they're much faster than before, which is why I say they would likely compete well, not cost him much of anything and not be dead weight when he upgrades. PCI cards are expensive and slow.
I don't disagree that the PCIe version of the X1550 is ok, but I wouldn't pay the price or hope for the performance of the PCI model, especially a card that is omitted by name in the Bioshock specs.
So unless you have something that shows the PCI version being stellar I'd say the latest and upcoming intergrated solutions are a better path, and then add a PCIe card later.