The real reason I have placed my GPU's under water was that the 4850x2 sounds like a rocketship taking off when the fans were on. Even with cpu core temps holding at 82C the gpu temps would rise about 1 or 2 degrees from where they were sitting idle, so the GPU temps I don't bother with.

As far as my radiator going into my res, I am a large tonnage chiller mechanic by trade so a little bit of thought went into that. I would much rather have a res full of the coldest water I can produce than to continually cool down a large heated pool. Next time you drive past some of those city buildings and see the "smoke" coming from the tops of them in the winter, that is more often than not their cooling tower. Though its an open loop the same principal applies. Pump -> Condenser of chiller -> Cooling tower -> Cooling tower sump. The added heat of the pump is negligible, the tower produces the coolest water possible and it sits in the sump ready to get pumped through.