Preemptive disclaimer that I don't work with GPUs a lot.

But I would imagine the 300W and 280W from your cards, part of that is coming from the risers. If you don't plug the risers into a heavy external supply but run them off your ATX, I'd allot at least 6A of 12V per riser. If the 300W/280W is straight external, they should all run off the DPS-1520 without issue. If that includes the socket power (from risers in this case), you could power the risers off the DPS-1520 and still only be at 80% capacity.

If, then, you ran your 6950 and motherboard off the ATX supply, I'd recommend probably at least a 500W unit. Maybe more depending on what processor/motherboard you're running, and what overhead you want to allow for.

What you could do, and an idea I've toyed around with but haven't tested yet, is actually get two server supplies in parallel load-balanced (I don't know enough about that particular model to know if they current-share or not, but I think most do) and get a picoPSU running off the 12V bus for your motherboard. Those things aren't terribly expensive, and run around 96% efficient. If you isolate your riser power from the motherboard power (either by using the recommended USB-style risers or cutting the 12V lines to the ribbon) you shouldn't overload anything on the picoPSU by trying to pull GPU current through it, and anything else requiring 12V (like GPU, or processor VRMs) would pull straight from your ~3KW 12V source.