For fans, I would look at the 1850RPM Scythe GTs, if you can ever find them in stock. Review
here.
OK, so lets say 300W for each GTX480, 130W for the i7 gives 1030W. Bump that up to a nice round 1100W for some OC headroom and that is what you will be trying to dissipate at load.
Looking at Skinnee's results for the RX360:
We can see that even with high speed fans, the 360 will only just shift our heat load with a 15C delta (not great). So we have to go for a bigger rad, but then we knew that already.
So, for an RX480 rad we take the 1100W and multiply by 3/4, this gives 825W. Looking at the table, 1800RPM fans can shift that with a delta of 15C. Better but still not great.
Now an RX480 and an RX240, take the 1100W and multiply by 3/6 (or 1/2), this gives 550W. Look at the table again, now your 1800RPM fans shift it with a 10C delta. Much better (same result as running 2 RX360s), and good performance.
For overkill, an RX480 and an RX360, multiply 1100W by 3/7, gives 471W. Now 1400RPM fans can shift the heat with a 10c delta, 1800RPM fans can manage roughly an 8C delta. Great performance.
You can go push pull for roughly a 30% performance boost (makes a single 480 viable with a 10C delta and a 480 - 360 combo a monster with a ~6C Delta), or add shrouds for roughly a 6% performance boost. Details
here.
Edit: And as Nick said, loop order doesn't matter other than res>pump. Water temperature rise is so small that it is far better to set the loop up in a way to give you the cleanest lines, least restriction and best flow.