For the longest time I couldn't find those drivers. However NVIDIA's 177.92 drivers do actually enable 3-Way SLI support for the D5400XS motherboard. So I can now run three cards in 3-Way SLI mode if I want to. I do have a 3-Way SLI bridge from my Striker II Formula that I can use for that since the D5400XS doesn't come with a 3-Way SLI bridge.
There are pros and cons to doing this. The Geforce GTX 280's cause me cold boot issues. It takes two or three attempts to start the system. Note: I haven't had that problem with any previous video cards I've used on the board such as the NVIDIA 8800GTX, 9800GX2, 8600GTS, etc. The other issue is the onboard sound fails to initialize and shows up as not installed in device manager occasionally. Lets' not forget that all my expansion slots would be gone at that point.
The ATI cards do not cause me the cold boot issues. They do however prevent me from using my LSI MegaRAID because the option ROM configuration of those boards is terrible just like the 9800GX2. So while there are dual GPUs on the card, the design isn't much more elegant than the NVIDIA GX2 style cards are. I couldn't use my LSI MegaRAID with the 3-Way SLI setup either, but only because all the slots will be taken by the cards. At least with the 4870 X2's I can get myself a PCI-Express sound card which is an appealing idea.
Forgive the stupid questions I'm about to ask. I don't own a Skulltrail and from the sounds of the problems your having I wouldn't get one even if I had the money.
If you go down to 2-280s in SLI, how much of a preformance hit do you see on games that DON'T go over 60 FPS all the time, maxed graphic settings.
What kind of OCing are you getting out of the skulltrail? Since it seems your diong more gaming on that system than anything else, how high are you pushing your quad cores? Or are you using 2 Dual Cores?