Wow, I definitely want to see more about this -- passive Sempy, here I come. Would be interesting to see some photos inside your case!
Well, semi-passive, I suppose. The cut-down Ninja is still big enough to fill almost 1/4 of the volume of the motherboard chamber, and it ended up about half way between the first and the second 120mm Tricool exhaust, which means there's a lot of airflow going through it even without directly mounting a fan to it.
I'll try to make some photos tomorrow when the big yellow lightbulb out in that strange big blue room is turned on again, I've only got a 1.3 Mpix cameraphone available at the moment, which is utter crap at low-light and not that great at other times, so still don't expect the quality to be stellar.
In my build, I wound up running the main ATX power cable through the HDD/PSU divider's oval opening (under the optical drives tray). It worked wonders for my cable roouting, as I have rather tall sound card and hardware modem card in the bottom 2 PCI slots. I'm also a bit tempted to tape over the rear 70mm square vent and the HDD chamber vents that are not directly under the hard disks in order to force more airflow over them and through the VGA card vent slots on the top. Has anyone tried this?
Going through that side is a good idea. I'll have to check whether my MCE150s will fit as it is, but a longish PCI card is definitely blocked in the last slot position as I have it now. Taping up vents to force a longer airflow path sounds like it will probably work, but you'll create a lower pressure inside the case so the exhaust fans will have slightly more work to do and might get a bit noisier, and/or need to be set a bit higher for adequate cooling.
I think I *may* have found a way to fit the NBF-47 on the motherboard, although the positioning will be a bit awkward. But the stock heatsink is *so* small, and gets so hot, that I think it might well be worth it. One thing that bothers me is that the northbridge is an old-fashioned flip-chip design with the silicon slab exposed, and I've destroyed enough old Athlons to be leery of that. The stock heatsink has a small foam surround to keep it reasonably level, somewhat like the round pads on the later athlons, but it's very firmly attached to the heatsink, and not usable on a 3d party one.
 With the cover on and just regular 2D video usage, it gets to what my uncalibrated finger judges as 40-50C or so, with the cover off it gets to 60-70. I don't even want to think what 3D video would push it to.