I don't really understand, what is PCIE3 x16 mode, PCIE4 X4 mode. I just saw the two PCIE slots on the motherboard support CrossFireX or SLI, they are just same slots, all x16 graphic card can fit into it. Can I just for example insert two GTX 560 and is that it?
The 970 chipset does support 2 X pci express 16 slots. Hoever, unlike the top end motherboards like the 990FX, these boards are limited on pci exress lanes.
What this is trying to tell you is if you populate one channel with a graphcis card, it will run at 16X speed. I think 990X offers X8/X8 when you use two graphcis cards.
So according to the specs given above, on your motherboard if you populate both X16 slots with video cards, one will run at 16X speed but the second one would be limited to 4X.
Its not an ideal situation for crossfire, which is why most enthusiasts would go for the 990FX motherboards so they could have full 16X/16X for crossfire or SLI.
Reading the reviews on newegg confirms this board runs at 16X/4X with sli, the 990X extreme3 is capable of 8X/8X.
But the 990FX Extreme3 for $130 looks pretty sweet.