Easy way to remember.
2 cards = 16x
3 or more = 8x
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1st 16x slot from the cpu =16x
2ed 16x slot from the cpu =8x
3ed 16x slot from the CPU= 16x
4th 16x slot from the CPU =8x
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If its only two slots on the motherboard. Then they are both 16x in speed (these days).
Can you tell a difference NO
(not yet at least maybe with 2 4870x2 cards but even then.....)
If your on a AMD chipset(AMDcpu) you could run your cards with no wires, and do everything on the 8x buss. No slow downs.
I kind of like running my cards one down and not on the top. Makes it easier to work inside the case
