Welcome to Procooling!
You've got an easy question: volume is completely irrelevant.
Once all the components in your PC have stabilized in temperature, that's when your system is "in balance" or "equilibrium", kinda like that water temp needle in your car, on a cold winter morning: it'll eventually go up.
A larger volume of water will only delay the time it takes to reach that equilibrium.
As for the pump, flow (gallons per hour, or whatever) is important, but even more so is the pressure (psi) or head (feet). You ought to read Dave Smith's guide, on amdmb.com .
The White Water is the greatest block to date (pending a few things we're working on
), and will work very well with an Eheim 1048, or whatever pump has an equivalent flow rate and pressure/head. You ought to look at the review of it on overclockers.com