First off, I am not 3D-apps/Video-Editing person. I do not know anything about those. Nonetheless, my tips will help you a bit.
To explain the following 2 lines from X7DA8 page,
"1 (x16) PCI-e slot
1 (x4) PCI-e (using x16) slot"
the mobo has 2 PCIE slots. The 1st slot has 16 lanes, the 2nd one has 4 lanes but it uses the same physical slot as 16-lane slot. In contrast, look at Supermicro h8dc8. You'll see that board has 2 independent 16-lanes PCIE slots. Basically, more lanes mean more data transport.
In SLI mode, one video card renders half of current scene, the 2nd card renders the other half. Since they transport data through SLI bridge, (I don't think) 2 independent 16-lane PCIEs matter much (i.e for gaming).
But without SLI bridge, I would think that having 2 independent 16-lane slots will start to matter for 3D-app.
As a side note, I am not a gamer but I did built a couple of gaming rigs using Supermicro H8DC8's and H8DCE's with dual videos in SLI configuration. These boards are server motherboards but I find them very stable for gaming too.