Answer is none.
PCIe is uses serial communication. So bit width is not relevent here. PCIe 16x just means it has 16 channels as opposed to PCIe 1x, meaning 1x is slower than 16x common sense login
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Original pci was parallel communication, and it was 32bit wide. There are later editions as PCI-X 64bit and PCI-X 2.0.
To have full understanding read:
http://www.hardwaresecrets.com/article/190