Hope you folks are still checking this thread.
I've learned much this summer, despite a 20-year involvement with this technology. And I say that because I've never spent a lot of time on the cooling issue until now.
First, it seems that different types of computer cases have different cooling and temperature "profiles". A mid-tower case has a warmer internal ambient than a full tower, and even the over-hyped CoolerMaster WaveMaster does not seem to pull the case out of that mid-tower pigeon-hole.
Second, I began to make some assumptions based on simple common sense about the signal-conductor architecture of Pentium-4 motherboards. I think these assumptions would extend to AMD mainboards as well.
The Northbridge chipset is connected to both the AGP and the memory modules, in addition to the CPU. Biggest among my assumptions was the idea that the hottest component in the box will adversely affect the idle temperatures of anything connected to it, and that those items most directly connected will have the biggest interaction of temperature readings. Therefore, if the AGP card is the hottest item in the box, cool down the AGP card and "their hearts and minds will follow."
My system consists of:
2.4C P-4 @ 2.88
ASUS P4P800 (standard)
ASUS v 9850 (nvidia) FX5950 Ultra 256mb
1 GB (512 X 2) OCZ "Gold, EL" PC4000 (DDR500 @ DDR480)
dual Hitachi 160gb SATA150 in RAID0 array
Sony DRU-700A and TDK 52X CDRW
Full-tower (vintage 1995) Gateway 2000 case
dual 120mm EverCool aluminum intake fans
dual 92mm ThermalTake Blue LED fans
CPU fan: Thermaltake 120mm Blue LED with clear-acrylic 92-to-120mm fan adapter
Thermaltake PIPE101 heatpipe heatsink
Creative Labs SB Audigy 2 ZS
Hauppauge PVR 250 capture + TV
AGP cooler: Zalman dual-heatpipe ZM80D with Zalman 80mm OP-1 option fan
Temperatures are monitored both by motherboard sensor (and ASUS software) and thermal sensors connected to a CoolerMaster Aerogate II fan-controller. The temperature sensors are mounted on the bottom of the CPU heatsink nearest the processor, bottom edge of the AGP heatsink next to the GPU, and bottom edge of the Northbridge (ASUS stock) heatsink, respectively.
While ASUS probe reports a CPU temp of 84F (varying with ambient, as does everything else), Probe also reports a "motherboard temp of 87F. The Coolermaster Aerogate sensors report 92.3F, 103.1F, and 101.3F for the CPU, AGP card, and chipset respectively.
Before I installed the Zalman AGP cooler, the FX5950 reported a GPU temperature between 106F and 110F. The chipset temperature was always one to two degrees F behind the AGP temp under all cooling regimes -- before and after adding the Zalman AGP cooler. That is, by adding the AGP cooler, the temperatures of both the AGP and the Northbridge dropped approximately the same number of degrees F.
104F is 40C. I prefer Fahrenheit, because it gives me a better understanding of temperatures in comparision with a room ambient which I have always measured in degrees F most of my life.
By the way. Those fancy-dancy copper HSF assemblies for the Northbridge are probably great. But for about $7 you can get a 40mm SUNON "mag-lev" fan that spins at 8,000 rpm with no perceptible noise (Yeah! believe it!) You can hang this thing over the AGP card with a Zalman arm bracket using a very long bolt to put the fan close the the stock motherboard cooler without replacing it with one of those $35 assemblies. It DOES help! And you don't need to putz around with the stock heatsink and run the risk of damaging something if your fingers slip!