I'd SERIOUSLY go for a single loop
several advantages are:
-maintenance.. its easier to drain and refill a single loop
-single reservoir... just a tiny difference in cost
-single (though large) radiator... cheaper than 2 smaller ones.
-safety... i'll explain this next.
first of all, a single extreme end loop will, like dual loops, use 2 pumps.
that's for two reasons. A) you will get much better flow trough all those blocks, especially (and I recommend doing this) if you buy the Swiftech NB-MAX block for your awesome motherboard (though a single good pump will still do perfectly fine in such a large loop) and B) if one pump fails, the other one will still keep your hardware from burning up, while if you had to do this with dual loops... well, who wants to pay for 4 pumps?
about the single huge rad. a GTX480 is capable of cooling the coolant with an 800W heatload (Skulltrail + QuadSLI + GeForce PhysX) to a mere 3C from ambient with 133CFM Ultra Kazes (any other 38mm thick fan in this CFM range will do about the same obviously) without shrouds. if you add the shrouds, you could drop to around 110 CFM or so, and since your heatload is lower, probably to 80-90CFM.
In short: the GTX480 rad wont be the weak link at all.
so there you have it. a single loop is technically superior to dual loops.
though dual loops have the advantage of easthetics. for example, you could use 2 differently colored coolants, different tubing sizes. but that suddenly doesnt seem as important, now does it?