What about using 2 radiators...

Here is what I was thinking for my setup:
500GPH pump/reservoir, the water comes out of the tank into the The Cooling Cube (Copper Core), then to the copper cpu block, then to the 5 1/8" x 11" Low Pressure Drop Cooler, then to the video card cooler and the chipset cooler before returning back into the reservoir.

The idea is to cool the water coming out of the CPU before going to the video card.

Sounds good.
Tell us how it does.

I don't think it would be so useful. As you have a high flowrate, why not trying to splice your water flow to the GPU? You can use a T-fitting and a small valve. You put the valve next to the T and turn it until the water exiting the GPU waterblock gets warm enough.