I'm not completely sure how much difference the duct makes by itself to graphics card temperatures.
The big improvement was in general system temperatures. I'd say it dropped CPU and hard drive temperatures significantly compared to not having it fitted. I'd guess it was a good 2-3c drop
The air flowing out of the duct was very hot and would normally have remained trapped inside the case.
I can't go back and compare as my replacement graphics card cooler is finally up and working.
It's been sitting around whilst I attempted to convince everyone it wasn't going to leak.
It's a Zalman Reserator 2 connected up to a ZM-GWB8800 GTS waterblock. It's using distilled water mixed with Zalman anti-corrosion coolant (apparently non conductive coolant is thicker and you need a stronger pump.)
The only departure from the kit is that I used proper hose clips (size 000 9.5mm to 12mm). These are a lot more secure than the metal clips supplied by Zalman.
It sits on a plastic grow-bag tray in case of leaks.
The Zalman water block is a solid piece of aluminium that weighs 420g! The Reserator 2 is quite solid as well. Most of it is a large aluminium heatsink apart from the rear of the unit which is a reservoir.
Installation is quite straightforward: Just assemble the tubes, tighten the hose clips and then part fill the reservoir.
After that you start and re-start the pump until all the air has come out of the system. Gently rocking the part filled reserator from side to side encourages the air bubbles to come out.
Installing the waterblock is hassle free. The main downside is that you have to use thermal pads for all components apart from the GPU core where I used Arctic Silver 5. The thermal pads are necessary because there is quite a large gap between the card components and the waterblock.
ZM-GWB8800 GTS Waterblock Base
When installed the waterblock looks like this:
ZM-GWB8800 GTS waterblock installed on Nvidia GF8800GTS 640mb card
The tubing is deliberately cut fairly long. It means that the case can be moved around without affecting the waterblock.
Waterblock installed in Antec Solo case
The tubing layout is a little different to what you see in many setups. I cut a hole in the side of the case so that the card can be removed without needing to disassemble the entire PC. The hole is filled with foam from some old hard drive packaging.
This pic shows the cooler in action.
Reserator 2 Installed And In Use
The PC now has only a single running fan (Scythe S-Flex SFF21D 800rpm), it also has a single hard drive (Samsung Spinpoint T 400gb in Growup Japan 2002c enclosure) plus the 5w pump in the reserator.
The Nesteq PSU is semi-fanless in ASM mode.
Overall it's a lot quieter than with the 8800GTS stock cooler installed (it had quite a high pitched tone).
The big improvement is when wearing headhones. With semi-open headphones like my Grado SR125's the 8800GTS fan was really annoying for some reason.
What's quite interesting is that noise from the PC is still quite noticeable.
The Scythe S-Flex fan is loudest followed by hard drive seeks and then a mixture of electrical noise from inside the PC and a faint gurgling noise (like a radiator!) coming from the reserator 2.
Room temperature is 21c approx.
After running for two hours at idle the graphics card core is at 37c.
For comparison the stock cooler at the same room temperatures idled between 49-50c.
The other case and system temperatures are noticeably lower than with the air cooled card also. The idle CPU temperature is 36c currently.
When comparing those temperatures please remember that this is with a custom BIOS so the card is idling at 100mhz rather than full 3D speeds.
8800GTS Idle Water Temperatures