To answer the second part of the second question.
the X something is the among of lanes there are in the PCI-E port.
Think of it as how wide the port is.
Here is a picture that should explain the main difference.
you will see some X8 ports that looks like X16. That is because it will support X16 cards, but can only use half the total bandwidth. The reason it only have half the bandwidth is because that half of the port is empty. It only has 8 lanes. The next 8 are empty.
Luckily most GPU's don't suffer in performance with half the bandwidth (the high performance once does). They keep on updating the PCI-E standard. We are at PCI-E 3.0 now. So we can put more data through less and less lanes. So an older GTX 680 might be suffering a little bit in performance on a PCI-E X8 2.0 port but are gonna perform at it's fullest on a X8 3.0 port (or a X16 2.0 port). Interesting ehh?
---------- Post added at 09:37 AM ---------- Previous post was at 09:35 AM ----------
Ohh, and PCI (or PCI 64) is a pretty much dead standard. PCI and PCI-E are not alike. They are just as different as firewire and USB