Asrock p67 Extreme4 Gen3 motherboard

With that midrange GPU i highly doubt you will notice a drop in performance.

I also highly doubt you would saturate both SSD's in RAID 0 all the time to be able to notice a drop in GPU performance.

Basically you would have to be enconding some HD videos while playing a game to notice a drop in performance - and that in itself would definately cause drops in performance.

Since this board has PCIE 3.0 slots, what do you think will happen with future video cards? The PLX chip claims it will allow all devices to work at max bandwidth, but I'm concerned if I get a new/future video card, there may be issues.

Well in the future - when a GPU can take advantage of ALL OF PCIE 3.0's bandwidth - i'm pretty sure your whole computer will be outdated to even take advantage of it. It would be like having a Pentium 3 1Ghz trying to feed a AMD 6990 GPU (even though this isn't obviously possibly, just an example).