What does the pci-e 16x 8x 4x mean?

I'm looking for an amd board that has crossfire and at least supports 1600mhz ram and I notice different boards say they have 2 pci-e x16 slots but have in parenthesis like (1 x16 or dual x8 x8) or 1 x16 and 1 x4. What does this mean and does it make much difference when crossfiring?
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  1. This is the number of data transfer lanes available. 16 is better, since it basically means more bandwidth for the GPUs to use, but benchmarks show that 8 lanes is enough for all but the most powerful dual GPU cards, and even those are only marginally bottlenecked (a few frames per second difference at most)
  2. 16x/4x will bottleneck any modern/decent CrossFireX setup, 8x/8x is fine.
  3. ^ But I'd take a step sideways or further. 1x16 is basically a single gpu configuration. dual 8x8 means that if you have a 2nd gpu, the slots will run at 8 lanes each (as opposed to 1 @ 16 lanes) and the 1x4 just means that the 3rd gpu will run at 4 lanes. Be sure that if you want to run crossfire or sli that you'd have at least the specs mentioned above and not just a 1x16 and 1x4.
  4. So having an x8 and x8 board would be fine crossfiring two 5770's or 5850's?
  5. Yes it would be fine, you'd see less than 4% performance penalties compared to if you used a 16x/16x board.
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