Each PCIe lane provides 250MBps (or is that Mbps? Sorry, I was watching UFC 92 tonight and my head is still swimming...) or 500MBps if its PCIe 2.0. The more lanes you have bundled together, the more data can be moved. What really matters is what card is plugged into the slot. Two weak PCIe2 cards won't be as fast as a fast PCIe1 card. The same card probably won't show any difference just by moving to PCIe2.
You also have to take into account how fast a card can use data. You can't double performance just by doubling the bandwidth to the card. The card itself needs to be able to handle it. The other side of the coin of course is that you don't cut the speed in half by cutting down the lanes. You will get a slight hit for some games, but most are fine at even PCIe1 8x.
1. Probably not, I have heard that some of the newer cards wont slow to that speed, sorry i dont know which ones but i would be very wary of trying it.
2. This review/article should help but please remember they are using older cards here and i have yet to see the same thing tested with newer cards http://www.tomshardware.com/reviews/pci-express-scaling...
3. Honest answer Dont know for sure but i wouldnt think it would make much differance.
Is this something you are thinking of doing then ? If you let us know what it is exactly you want to do and tell us Motherboard and cards etc we can probably answer the question better.
@123urpked. AGP in any variant simply can not take PCI Express cards. Sounds like your referring to a PCI Express x4 slot with a X16 connector. In that case PCI Express v2 would make a fair difference as the PCI Express v2 is effectively twice as fast as v1 giving you a PCI Express x8 v1 slot. That is in theory and in theory it shouldn't slow down a high end card by that much.
I hope that helps clear things up.
PS: With PCI Express the X number is the number of active data lanes. The v number is the version. v1 can do 250Mbps per lane and v2 can do 500Mbps per lane.