On the motherboard and CPU side, even though the X99 chipset had some more cost around it at the moment, the one very nice feature is that more flexible assigning PCI lanes to your graphics cards. Which means instead of 4x4x4x4, you could get 16x8x8x8. That means your primary Titan (x16) could load the scene into memory and start rendering quicker, then the other cards would join in as they loaded up the scene. If you do a lot of lighting and material checking, that could be worth the investment.
Whatever you decide, seems like it will be a beastly machine. (I love the idea of a dual Titan Z setup personally
