(sorry for this freaking long post, didn't realize it as i typed it...)
2.0 is like the latest version of the pci-e x16 slot (the standard slot for gfx cards nowadays). a 2.0 slot allows twice the bandwidth of 1.0 slots, but it shouldn't really matter that 2.0 allows that because (i believe) there are no current gfx cards that can saturate the bandwidth available from even a 1.0 slot.
so basically, you can take the highest end gfx cards currently out right now and put one in a 1.0 slot, and one in a 2.0 slot, they should perform pretty much the same.
and 2.0 cards are backwards compatible with 1.0 slots, just so you know.
now, when you see like x8/x8, that would be referring to running dual gfx cards (and x8/x8 support being found on like amd's x/gx chipsets and intel's p55 chipset boards). when you're running just one gfx card, all the bandwidth from an x16 slot is available for that one card. if you run dual gfx cards, the bandwidth that would normally be available to just one card from the x16 slot would now be split in half (as in x16 -> x8, half), for the second card in the second slot.
there are also boards, like amd's fx chipset and intel's x58 chipset boards, that will run dual gfx cards, offering the full bandwidth from the two x16 slots being used to both cards (as in x16/x16).
so you might say, well if boards that only support dual cards running at x8/x8 only offer half the bandwidth to each slot, then both my cards will only run half as fast as if they were in a board that supported dual cards at x16/x16 offering the full bandwidth available from both slots.
well, it's been tested, compared and shown that whether you're running dual cards in x16/x16 or x8/x8, there would in fact (on average) be a small difference (sometimes depending on the game too) but it would really be negligible. why? it kinda goes back to what i said above, (again, i think) there are currently no cards that can saturate the available bandwidth.
atleast that's the way i've gathered it from reading a few articles and whatnot on this before. i have sort of like a hacked up explanation in my mind that i understand for myself, so this is me basically trying to put that in words lol.
ofcourse, anyone is welcome to correct me if there's something wrong though
, i try...