I am looking at options for a quieter media PC.

Is it possible to underclock a 6750 or an x2 4400 and run it with the CPU fan disconnected? If not, is there another 775 CPU that might work this way?

Now that video cards have usable video acceleration (VDPAU) I will start needing a lot less CPU; I'll probably pick up a fanless NVidia 8500 too.

Thanks,
Mark

No way with the stock cooler but if the AMD is 65 nm both should do fine passively cooled by a fanless Ninja, mini ninja or TR HR01Plus. This requires good free airflow in your case which you probably already know.

is that X2 4400+ 65nm Brisbane? I'm not sure it would be impossible with stock heatsink, provided you have some minimal case airflow, you're willing to go down all the way to 1000Mhz and are additionally undervolting (to less than what CnQ would set at that frequency). TDP should be less than 20W in this case.
I guess the same would apply to the 6750, drop FSB from 1333 to 1066 (or even 800), drop multi from 8 to 6 and it should run at pretty low voltage - without undervolting it will never run fanless.
Neither cpus are probably ideal, as the 45nm versions should be better suited for this, but if you already have them you could always try.
Both should be able to run without a fan at idle easily, so with decent fan regulation it might not be worth bothering really trying to achieve truly fanless under load.

For a couple of months I ran E6750 (Xeon 3075 to be exact) passively on Ninja (with 120mm fans nearby, ok). I alsa OCed it to 3,4 @ stock V. But I wouldn't really run it as it is with stock cooler passively. At least a mini Ninja would be required, or as said before HR-01 or similar. And some decent airflow nearby. The CPU is great, sure, but it's not nearly as cool as newer 45 nm CPUs.

Agreed. Fanless can be done with the 4200 at stocks speeds and a fairly large heatsink.

Currently using X2 4200+ @2.4GHz 1.050v (default 2.2GHz 1.25v) cooled by passive Deepcool Ice Blade Pro without any problem on my rig

I do not understand people that use the word passive when they remove the CPU Fan but they have one or more case fans.
Is much better to remove the case fan and use low sped CPU fan, you will get better temperature for CPU and north bridge and on the same time less noise.
For a CPU to work passive you need a big heatsink and not more than 20W at load. Even so with no fans you will need a heatsink with the same size for north bridge since the best north bridge uses about 16W at load.
You can see my system that is almost passive I still need a small fan for the north bridge.

Agreed

I don't agree. Sure, you need a large heatsink designed with passive use in mind (like the Ninja 2), but you can cool a far more powerhungry CPU than 20W. I cool my 89W X2 4200+ using no fan what so ever (0 case fans + Pico). Sure, it is as undervolted as can be and the temp shoots up to 65 degrees when under load, but it works:)

(Btw, who uses a Northbridge that use 16W in a silent computer these days anyways? 40mm fans is about the hardest thing to keep quiet...)