I am currently using Asus K8V-X SE which is terrible, because it has some AGP bug and sometimes X crashes at start. None of the reviews pointed that out, just like in one of discussions someone wrote about non-functional Wifi in another motherboard.
Now I want to buy something stable. I am thinking about P35 chipset, maybe from Asus or Gigabyte or MSI.
I want to use Radeon with open source drivers only (I am developing OpenGL game so 3D is important, so r600 is not an option now) and passive cooler like Scythe Ninja.
Nope, it helped at least for my nforce3 board. Then AGP was definitely more stable. Take a look into your BIOS.
Btw. currently P35 is oudated, go for P45 it you buy a new board. Asus+Gigabyte boards can be flashed without extra flash tool (already integrated), MSI needs DOS/Win tool for that, thats basically the main difference. Also I would choose a board with lots of PCI slots, but you may not need so many...
You may also consider G45, then you can use the opensource driver from intel - you can later add a dedicated card whenever you need it. It is basically the same price, just mainly used in micro atx board, so usuallly at max 2 pci slots