At this time, we don't know what slots the Intel PowerMac will have. It is possible that Apple will throw in one PCI-X slot for backwards compatibility. In that case, your answer might be "yes", depending on other factors (such as voltage compatibility).
Historically, Apple hasn't been good about supporting expansion options that they've moved away from. My guess is the new MacPro will have too-few PCIe slots and no PCI-X slots, even though it will have room for more of both.