Thanks for all the replies!
It will only be a single 12-core Xeon (+32GB 1600MHz RAM) and a single GTX Titan to begin with. I'm guessing the pricetag will be around $6.000-$7.000. I will add the other stuff one-by-one as I can afford them. I'm hoping this is possible? To buy a type of cooling system now that can be added to later on, and not having to replace it (when adding another Xeon and 2nd, 3rd and 4th Titan).
I'll be using this system mainly for 3D simulations (realflow, fumefx etc.), rendering (cpu and gpu rendering) and some VFX in After Effects/Nuke.
The reason I went for the Xeons (instead of a i7, which I considered initially) is because I'll be using Maxwell Render: http://www.maxwellrender.com/benchwell
The 4 GTX Titans will be for Octane Render: http://boxxblogs.blogspot.com/2013/06/gtx-780-gpu-rendering-benchmark-with.html
One thing though, according to the FAQ on Octanes site (http://render.otoy.com/faqs.php
, under Does Octane Render take advantage of SLI?) it is recommended I turn off SLI when rendering. So does this mean I should skip SLI cables?
Anyway, I've looked into these for cooling:
CORSAIR Hydro Series H110 Water Cooler
COOLER MASTER Hyper 212 EVO RR-212E-20PK-R2 Continuous Direct Contact 120mm Sleeve CPU Cooler
Arctic Silver 5 High-Density Polysynthetic Silver Thermal Compound AS5-3.5G
But, like I said, I have no experience with building a PC. So I have no idea what exactly is needed. If anyone can tell me exactly how many of each to get, and for which component, that would be VERY helpful!