I'm sure this issue has been beaten to death by many people but I just wanted to relate my own experience since I still occasionally see people asking about it in threads, wondering if there is a performance hit or high-temperature instability. For the many who I'm sure consider this a solved-problem, you can ignore this, I just wanted to add a little clarification for those who still occasionally ask about it. And post this to contribute to the forum which has helped me out a couple times.
The issue I am talking about is the one that has been brought up many times, the northbridge fan on the nf4 ultra-d is very noisy but also unfortunately placed right under the end of the pci-express slot1, which is the only one that can run at 16x.
When I got this motherboard the first thing I wanted to do was replace the noisy fan with a passive cooler (larger heatsink and no fan), but I didn't want to have to build a custom one or bend up an existing one to get it to fit. So I figured I would just move the video card to the other full-size pci-express slot, then I learned that you have a choice between running (slot1 16x - slot4 2x) or switching the jumpers to sli-mode and getting (slot1 8x - slot4 8x).
My concern was that at 8x I would take a performance hit by limiting the bandwidth available to the video card, as others have said, this is not an issue.
listed below are benchmark scores and northbridge temperature readings comparing-
(video card in slot1 at 16x - stock northbridge cooler) vs (video card in slot4 at 8x - passive northbridge cooler)
the northbridge cooler I used is the zalman zm-nb47j heatsink, I would suspect other heatsinks of similar size would also be suitable, as for air circulation, the fan in my power supply is almost always off, even at load, as is the cpu fan (CNPS7000B-ALCU). So the only real air movement comes from a cheap 120mm fan in the front blowing in and an identical fan on the back blowing out, and the vga cooler which just blows hot air into the center of the case, I would suspect the efficiency of a passive northbridge cooler in this scenario is largely dependent on case air flow, I never put a whole lot of work into mine and it seems more than sufficient.
quake4 - necro666
(slot1) 59.1 fps
(slot4) 59.2 fps
quake3 - four
(slot1) 276.5 fps
(slot4) 279.4 fps
FarCry - Pier, demo: 1.tmd
Score = 60.41 FPS (Run 1)
Score = 60.07 FPS (Run 2)
Average score = 60.24 FPS
Score = 62.30 FPS (Run 1)
Score = 62.07 FPS (Run 2)
Average score = 62.18 FPS
for some reason my scores actually went up a tiny bit on the (8x slot4) as opposed to the (16x slot1), why this happens I do not know but I read a couple other people having similar experiences on other forums. As for the temperatures, the at-load temp dropped with the passive cooler.
so for those being annoyed by the noise of the northbridge (and not running two video cards) you can move to slot4 and swap in a northbridge heatsink and as long as your case air flow isn't terrible you should be fine.