To clarify, by "x99," do you mean LGA2011-3? That's the socket wikipedia associates with the hardware.
And as for Arc, it looks like they're a great option for video encoding. I'm actually using QuickSync already on my Celeron processor which has helped. From what I can understand, it looks like QuickSync is basically the same processor on all of the Arc cards, so I can just go with the cheapest card if I don't plan to use much of the other features? Looking like an A380 can be had for $100 or so.