Will one 360mm rad be enough to cool a RIVBE, 290X, and a 4930K?
if thickness is 30mm or more and you have decent fans on it, yup
Didn't you already ask this? I'd include at least 120mm more of radiator space, but if you have nice fans, and a thick rad with high density, then you might be able run it at stock clocks with a 120.3. However, I'd recommend a 120.3 and 120.2 for overclocking room.
So, would a triple 320 and a dual 140 work to cool the 290X and 4930K if I slide a TEC in?
No need for a TEC; it won't improve your temps enough to merit the power draw. Yes, a 120.3 and 140.2 is more than enough - not sure why you're cooling your mobo, though.
I want to keep the VRMs cool.
That's what the heatsink is there for; no real need for a waterblock if the passive (silent) heatsinks are enough.
I disagree, if the motherboard is cooled then the ambient temperature within the case is reduced. I acknowledge that the VRMs, MOSFETs and bridge never get hot enough to warrant water-cooling, but they do produce some heat and do increase the ambient temperature slightly. If the loop can incorporate a motherboard waterblock, and the budget is adequate, I think the VRMs should be cooled, especially if heavy overclocking will be done.
Okay, what about if I'm cooling just the 290X and the 4930K? Will a 120.3 be enough without the mobo block?
Yes.
General rule of thumb. One 120mm Rad for each part being cooled, plus one more for good measure. 120.3 is perfect.