2 i'm 99% sure i'm going with
Grats on your Titans! I ordered one, waiting for preorders to come up again for my second. Anyway to the task at hand. You would probably want more radiator space, as I'm assuming you want to overclock those cards. If each card has about 250 W of heat without an OC, you already need 1000W of radiator space to cool it, A good rule of thumb is probably 120mm for every 100W. So you want more like 10.120mm radiator space (or thicker rads with push pull). And that's before including your CPU. I'd probably suggest you do a dual loop/pump design, with 2 GPUs on one loop, and two GPUs and the CPU on the other. That's if you want to be able to run the system with heavy OCs (and given your system it would be sacrilege not to
900D looks good. Any idea on availability?
If you need a large case now for cooling all that, get a Mountain Mods, Little Devil, or Caselabs case. None of them are cheap, but they are the guys to go to for big water cooling.