Not quite. Even though they have similar names, they really have nothing to do with each other. PCIe comes in several speed varieties including x1, x2, x4, x16, and x32. x64 is also in the works. A regular PCI slot does not work on the same bus as PCIe and works completely differently.
Considering the current PCIe slots are proportional in size to their speed, do you think the x128 will be the same, or do you think they'll be able to keep the size down? That would be a huge slot....