What exactly is this computer going to be used for, with how many monitors at what refresh rate and resolution?
In pretty much any scenario, you're better off just taking out your wallet and setting fire to it.
That being said, each titan will draw about 250w, so I'd look at an 850w power supply of reliable quality.
Hey again! I found a calculator online and found my wattage, never knew they had those. It even did it for my cathodes and led controllers.