I'm using 1850 Gt's but unvolted to 7v,the radiator i'm using is BI SR1-360 which has deep shroud built into one side of the radiator & i have fitted the fans to this side (pushing air).
Now i'm only cooling my cpu, with my previous cooler (Meghalems) it would idle at 38-40c i7 920 @ 4.00Ghz & load at 75-78c when running intel burn test.With a koolance 370 block & the above radiator it idles @ 33-35c but load is now down to 60-61 & also the weather has warmed up a lot later aswell.
My CPU with a H50 used to be 32-34 Idle, 60-62 with load
With the HF and 360 Rad (CPU is first in the loop to be cooled) Idles 34-36, Load 56-58Degrees C - nowhere near a drastic a drop in temps as you.
My GPU's however don't seem to move from 31Degrees C lol