Looks like all is not well in Intel 915 land, where there are some problems with certain Intel 915 boards
with certain ATI and Nvidia cards.
Due to "PCI Espresso Bongo stability" problems, the Intel 915 chipset may not be quite ready for PCI Express yet. i925-based boards do not suffer from this problem and should handle PCI Express no problem. As always, ATI, Nvidia and Intel are currently shouting at each other.