I can tell you my numbers for illustration, it helps if you can measure your ambient and water temps.
mine with 1 x 360 rad, mcp355 and i7 950 with vcore 1.34 OCed to 4.4 with HK block in single loop. After an hour of full load, ambient 24C, water temp 30C, core temp avg 68C.
So basically delta ambient to water is 6C, and adding more rads will only decrease that 6C, nothing else. The biggest problem is delta from core to water which is 38C (68C-water temp 30C), changing tim/block may help a little with that delta, but already using best tim and one best waterblocks. Changing the rads wont help with that. So look at it this way, ambient is 24C, water to core temp (basically gradient from core to IHS) is 38C, so no matter what I cant get below 62C load, even with 50 rads and powerful fans.
So now I add a second 360 rad, at best I can get another 3C, cut that 6C in half. However I am using relatively quiet fans 1200-1500 rpms. Instead of getting a second rad, I could have just gotten more powerful fans, but I dont want noise.
First I added 295gtx to single loop, my water temp only increased 1C, since at idle it only puts about about 80W or so (at gpu load, yes delta is way up there 15C or so). But at cpu load only, had 7C delta with 1200 rpm fans, then got second rad with quiet fans 1200 rpms, my delta is now ~3.5-4C from water to ambient at cpu only load.
So after 2x360 rads (without gtx 295) my delta water to air dropped from 6C to 3C, so temps 3C better with second rad. With added GTX at idle which adds 1C to water temp, I only got 2C total better temps with adding a second rad with adding gtx. However with both cpu and gpu fully loaded, that second rad becomes very noticable, since cuts my water delta down from 15C to about half that.